diff --git a/config/llvm.m4 b/config/llvm.m4
index fa4bedd9370..9d6fe8199e3 100644
--- a/config/llvm.m4
+++ b/config/llvm.m4
@@ -4,7 +4,7 @@
 # -----------------
 #
 # Look for the LLVM installation, check that it's new enough, set the
-# corresponding LLVM_{CFLAGS,CXXFLAGS,BINPATH} and LDFLAGS
+# corresponding LLVM_{CFLAGS,CXXFLAGS,BINPATH,LIBS}
 # variables. Also verify that CLANG is available, to transform C
 # into bitcode.
 #
@@ -55,7 +55,7 @@ AC_DEFUN([PGAC_LLVM_SUPPORT],
 
   for pgac_option in `$LLVM_CONFIG --ldflags`; do
     case $pgac_option in
-      -L*) LDFLAGS="$LDFLAGS $pgac_option";;
+      -L*) LLVM_LIBS="$LLVM_LIBS $pgac_option";;
     esac
   done
 
diff --git a/config/programs.m4 b/config/programs.m4
index c73d9307ea8..e57fe4907b8 100644
--- a/config/programs.m4
+++ b/config/programs.m4
@@ -290,8 +290,8 @@ AC_DEFUN([PGAC_CHECK_LIBCURL],
   pgac_save_LDFLAGS=$LDFLAGS
   pgac_save_LIBS=$LIBS
 
-  CPPFLAGS="$LIBCURL_CPPFLAGS $CPPFLAGS"
-  LDFLAGS="$LIBCURL_LDFLAGS $LDFLAGS"
+  CPPFLAGS="$CPPFLAGS $LIBCURL_CPPFLAGS"
+  LDFLAGS="$LDFLAGS $LIBCURL_LDFLAGS"
 
   AC_CHECK_HEADER(curl/curl.h, [],
 				  [AC_MSG_ERROR([header file <curl/curl.h> is required for --with-libcurl])])
diff --git a/configure b/configure
index 6d7c22e153f..8a535da6b7a 100755
--- a/configure
+++ b/configure
@@ -5194,7 +5194,7 @@ fi
 
   for pgac_option in `$LLVM_CONFIG --ldflags`; do
     case $pgac_option in
-      -L*) LDFLAGS="$LDFLAGS $pgac_option";;
+      -L*) LLVM_LIBS="$LLVM_LIBS $pgac_option";;
     esac
   done
 
@@ -9436,12 +9436,12 @@ fi
   # Note the user could also set XML2_CFLAGS/XML2_LIBS directly
   for pgac_option in $XML2_CFLAGS; do
     case $pgac_option in
-      -I*|-D*) CPPFLAGS="$CPPFLAGS $pgac_option";;
+      -I*|-D*) INCLUDES="$INCLUDES $pgac_option";;
     esac
   done
   for pgac_option in $XML2_LIBS; do
     case $pgac_option in
-      -L*) LDFLAGS="$LDFLAGS $pgac_option";;
+      -L*) LIBDIRS="$LIBDIRS $pgac_option";;
     esac
   done
 fi
@@ -9666,12 +9666,12 @@ fi
   # note that -llz4 will be added by AC_CHECK_LIB below.
   for pgac_option in $LZ4_CFLAGS; do
     case $pgac_option in
-      -I*|-D*) CPPFLAGS="$CPPFLAGS $pgac_option";;
+      -I*|-D*) INCLUDES="$INCLUDES $pgac_option";;
     esac
   done
   for pgac_option in $LZ4_LIBS; do
     case $pgac_option in
-      -L*) LDFLAGS="$LDFLAGS $pgac_option";;
+      -L*) LIBDIRS="$LIBDIRS $pgac_option";;
     esac
   done
 fi
@@ -9807,12 +9807,12 @@ fi
   # note that -lzstd will be added by AC_CHECK_LIB below.
   for pgac_option in $ZSTD_CFLAGS; do
     case $pgac_option in
-      -I*|-D*) CPPFLAGS="$CPPFLAGS $pgac_option";;
+      -I*|-D*) INCLUDES="$INCLUDES $pgac_option";;
     esac
   done
   for pgac_option in $ZSTD_LIBS; do
     case $pgac_option in
-      -L*) LDFLAGS="$LDFLAGS $pgac_option";;
+      -L*) LIBDIRS="$LIBDIRS $pgac_option";;
     esac
   done
 fi
@@ -12723,8 +12723,8 @@ if test "$with_libcurl" = yes ; then
   pgac_save_LDFLAGS=$LDFLAGS
   pgac_save_LIBS=$LIBS
 
-  CPPFLAGS="$LIBCURL_CPPFLAGS $CPPFLAGS"
-  LDFLAGS="$LIBCURL_LDFLAGS $LDFLAGS"
+  CPPFLAGS="$CPPFLAGS $LIBCURL_CPPFLAGS"
+  LDFLAGS="$LDFLAGS $LIBCURL_LDFLAGS"
 
   ac_fn_c_check_header_mongrel "$LINENO" "curl/curl.h" "ac_cv_header_curl_curl_h" "$ac_includes_default"
 if test "x$ac_cv_header_curl_curl_h" = xyes; then :
@@ -16658,7 +16658,7 @@ fi
 
 if test "$with_icu" = yes; then
   ac_save_CPPFLAGS=$CPPFLAGS
-  CPPFLAGS="$ICU_CFLAGS $CPPFLAGS"
+  CPPFLAGS="$CPPFLAGS $ICU_CFLAGS"
 
   # Verify we have ICU's header files
   ac_fn_c_check_header_mongrel "$LINENO" "unicode/ucol.h" "ac_cv_header_unicode_ucol_h" "$ac_includes_default"
@@ -18876,7 +18876,7 @@ Use --without-tcl to disable building PL/Tcl." "$LINENO" 5
     fi
     # now that we have TCL_INCLUDE_SPEC, we can check for <tcl.h>
     ac_save_CPPFLAGS=$CPPFLAGS
-    CPPFLAGS="$TCL_INCLUDE_SPEC $CPPFLAGS"
+    CPPFLAGS="$CPPFLAGS $TCL_INCLUDE_SPEC"
     ac_fn_c_check_header_mongrel "$LINENO" "tcl.h" "ac_cv_header_tcl_h" "$ac_includes_default"
 if test "x$ac_cv_header_tcl_h" = xyes; then :
 
@@ -18945,7 +18945,7 @@ fi
 # check for <Python.h>
 if test "$with_python" = yes; then
   ac_save_CPPFLAGS=$CPPFLAGS
-  CPPFLAGS="$python_includespec $CPPFLAGS"
+  CPPFLAGS="$CPPFLAGS $python_includespec"
   ac_fn_c_check_header_mongrel "$LINENO" "Python.h" "ac_cv_header_Python_h" "$ac_includes_default"
 if test "x$ac_cv_header_Python_h" = xyes; then :
 
diff --git a/configure.ac b/configure.ac
index c2877e36935..e72201e679b 100644
--- a/configure.ac
+++ b/configure.ac
@@ -1103,12 +1103,12 @@ if test "$with_libxml" = yes ; then
   # Note the user could also set XML2_CFLAGS/XML2_LIBS directly
   for pgac_option in $XML2_CFLAGS; do
     case $pgac_option in
-      -I*|-D*) CPPFLAGS="$CPPFLAGS $pgac_option";;
+      -I*|-D*) INCLUDES="$INCLUDES $pgac_option";;
     esac
   done
   for pgac_option in $XML2_LIBS; do
     case $pgac_option in
-      -L*) LDFLAGS="$LDFLAGS $pgac_option";;
+      -L*) LIBDIRS="$LIBDIRS $pgac_option";;
     esac
   done
 fi
@@ -1152,12 +1152,12 @@ if test "$with_lz4" = yes; then
   # note that -llz4 will be added by AC_CHECK_LIB below.
   for pgac_option in $LZ4_CFLAGS; do
     case $pgac_option in
-      -I*|-D*) CPPFLAGS="$CPPFLAGS $pgac_option";;
+      -I*|-D*) INCLUDES="$INCLUDES $pgac_option";;
     esac
   done
   for pgac_option in $LZ4_LIBS; do
     case $pgac_option in
-      -L*) LDFLAGS="$LDFLAGS $pgac_option";;
+      -L*) LIBDIRS="$LIBDIRS $pgac_option";;
     esac
   done
 fi
@@ -1177,12 +1177,12 @@ if test "$with_zstd" = yes; then
   # note that -lzstd will be added by AC_CHECK_LIB below.
   for pgac_option in $ZSTD_CFLAGS; do
     case $pgac_option in
-      -I*|-D*) CPPFLAGS="$CPPFLAGS $pgac_option";;
+      -I*|-D*) INCLUDES="$INCLUDES $pgac_option";;
     esac
   done
   for pgac_option in $ZSTD_LIBS; do
     case $pgac_option in
-      -L*) LDFLAGS="$LDFLAGS $pgac_option";;
+      -L*) LIBDIRS="$LIBDIRS $pgac_option";;
     esac
   done
 fi
@@ -1944,7 +1944,7 @@ fi
 
 if test "$with_icu" = yes; then
   ac_save_CPPFLAGS=$CPPFLAGS
-  CPPFLAGS="$ICU_CFLAGS $CPPFLAGS"
+  CPPFLAGS="$CPPFLAGS $ICU_CFLAGS"
 
   # Verify we have ICU's header files
   AC_CHECK_HEADER(unicode/ucol.h, [],
@@ -2344,7 +2344,7 @@ Use --without-tcl to disable building PL/Tcl.])
     fi
     # now that we have TCL_INCLUDE_SPEC, we can check for <tcl.h>
     ac_save_CPPFLAGS=$CPPFLAGS
-    CPPFLAGS="$TCL_INCLUDE_SPEC $CPPFLAGS"
+    CPPFLAGS="$CPPFLAGS $TCL_INCLUDE_SPEC"
     AC_CHECK_HEADER(tcl.h, [], [AC_MSG_ERROR([header file <tcl.h> is required for Tcl])])
     CPPFLAGS=$ac_save_CPPFLAGS
 fi
@@ -2381,7 +2381,7 @@ fi
 # check for <Python.h>
 if test "$with_python" = yes; then
   ac_save_CPPFLAGS=$CPPFLAGS
-  CPPFLAGS="$python_includespec $CPPFLAGS"
+  CPPFLAGS="$CPPFLAGS $python_includespec"
   AC_CHECK_HEADER(Python.h, [], [AC_MSG_ERROR([header file <Python.h> is required for Python])])
   CPPFLAGS=$ac_save_CPPFLAGS
 fi
diff --git a/src/Makefile.global.in b/src/Makefile.global.in
index 04952b533de..8b1b357beaa 100644
--- a/src/Makefile.global.in
+++ b/src/Makefile.global.in
@@ -254,7 +254,7 @@ CPP = @CPP@
 CPPFLAGS = @CPPFLAGS@
 PG_SYSROOT = @PG_SYSROOT@
 
-override CPPFLAGS := $(ICU_CFLAGS) $(LIBNUMA_CFLAGS) $(LIBURING_CFLAGS) $(CPPFLAGS)
+override CPPFLAGS += $(ICU_CFLAGS) $(LIBNUMA_CFLAGS) $(LIBURING_CFLAGS)
 
 ifdef PGXS
 override CPPFLAGS := -I$(includedir_server) -I$(includedir_internal) $(CPPFLAGS)
diff --git a/src/backend/jit/llvm/Makefile b/src/backend/jit/llvm/Makefile
index e8c12060b93..68677ba42e1 100644
--- a/src/backend/jit/llvm/Makefile
+++ b/src/backend/jit/llvm/Makefile
@@ -31,7 +31,7 @@ endif
 # All files in this directory use LLVM.
 CFLAGS += $(LLVM_CFLAGS)
 CXXFLAGS += $(LLVM_CXXFLAGS)
-override CPPFLAGS := $(LLVM_CPPFLAGS) $(CPPFLAGS)
+override CPPFLAGS += $(LLVM_CPPFLAGS)
 SHLIB_LINK += $(LLVM_LIBS)
 
 # Because this module includes C++ files, we need to use a C++
diff --git a/src/bin/initdb/Makefile b/src/bin/initdb/Makefile
index 997e0a013e9..c0470efda92 100644
--- a/src/bin/initdb/Makefile
+++ b/src/bin/initdb/Makefile
@@ -20,7 +20,7 @@ include $(top_builddir)/src/Makefile.global
 # from libpq, else we have risks of version skew if we run with a libpq
 # shared library from a different PG version.  Define
 # USE_PRIVATE_ENCODING_FUNCS to ensure that that happens.
-override CPPFLAGS := -DUSE_PRIVATE_ENCODING_FUNCS -I$(libpq_srcdir) -I$(top_srcdir)/src/timezone $(ICU_CFLAGS) $(CPPFLAGS)
+override CPPFLAGS := -DUSE_PRIVATE_ENCODING_FUNCS -I$(libpq_srcdir) -I$(top_srcdir)/src/timezone $(CPPFLAGS) $(ICU_CFLAGS)
 
 # We need libpq only because fe_utils does.
 LDFLAGS_INTERNAL += -L$(top_builddir)/src/fe_utils -lpgfeutils $(libpq_pgport) $(ICU_LIBS)
diff --git a/src/common/Makefile b/src/common/Makefile
index 1e2b91c83c4..2c720caa509 100644
--- a/src/common/Makefile
+++ b/src/common/Makefile
@@ -163,7 +163,7 @@ libpgcommon_shlib.a: $(OBJS_SHLIB)
 # The JSON API normally exits on out-of-memory; disable that behavior for shared
 # library builds. This requires libpq's pqexpbuffer.h.
 jsonapi_shlib.o: override CPPFLAGS += -DJSONAPI_USE_PQEXPBUFFER
-jsonapi_shlib.o: override CPPFLAGS += -I$(libpq_srcdir)
+jsonapi_shlib.o: override CPPFLAGS := -I$(libpq_srcdir) $(CPPFLAGS)
 
 # Because this uses its own compilation rule, it doesn't use the
 # dependency tracking logic from Makefile.global.  To make sure that
diff --git a/src/interfaces/libpq-oauth/Makefile b/src/interfaces/libpq-oauth/Makefile
index 270fc0cf2d9..682f17413b3 100644
--- a/src/interfaces/libpq-oauth/Makefile
+++ b/src/interfaces/libpq-oauth/Makefile
@@ -24,7 +24,7 @@ NAME = pq-oauth-$(MAJORVERSION)
 override shlib := lib$(NAME)$(DLSUFFIX)
 override stlib := libpq-oauth.a
 
-override CPPFLAGS := -I$(libpq_srcdir) -I$(top_builddir)/src/port $(LIBCURL_CPPFLAGS) $(CPPFLAGS)
+override CPPFLAGS := -I$(libpq_srcdir) -I$(top_builddir)/src/port $(CPPFLAGS) $(LIBCURL_CPPFLAGS)
 
 OBJS = \
 	$(WIN32RES)
diff --git a/src/interfaces/libpq/Makefile b/src/interfaces/libpq/Makefile
index 47d67811509..da6650066d4 100644
--- a/src/interfaces/libpq/Makefile
+++ b/src/interfaces/libpq/Makefile
@@ -24,7 +24,7 @@ NAME= pq
 SO_MAJOR_VERSION= 5
 SO_MINOR_VERSION= $(MAJORVERSION)
 
-override CPPFLAGS :=  -I$(srcdir) $(CPPFLAGS) -I$(top_builddir)/src/port -I$(top_srcdir)/src/port
+override CPPFLAGS := -I$(srcdir) -I$(top_builddir)/src/port -I$(top_srcdir)/src/port $(CPPFLAGS)
 ifneq ($(PORTNAME), win32)
 override CFLAGS += $(PTHREAD_CFLAGS)
 endif
diff --git a/src/pl/plpython/Makefile b/src/pl/plpython/Makefile
index f959083a0bd..25f295c3709 100644
--- a/src/pl/plpython/Makefile
+++ b/src/pl/plpython/Makefile
@@ -11,7 +11,7 @@ ifeq ($(PORTNAME), win32)
 override python_libspec =
 endif
 
-override CPPFLAGS := -I. -I$(srcdir) $(python_includespec) $(CPPFLAGS)
+override CPPFLAGS := -I. -I$(srcdir) $(CPPFLAGS) $(python_includespec)
 
 rpathdir = $(python_libdir)
 
diff --git a/src/pl/tcl/Makefile b/src/pl/tcl/Makefile
index ea52a2efc22..dd57f7d694c 100644
--- a/src/pl/tcl/Makefile
+++ b/src/pl/tcl/Makefile
@@ -11,7 +11,7 @@ top_builddir = ../../..
 include $(top_builddir)/src/Makefile.global
 
 
-override CPPFLAGS := -I. -I$(srcdir) $(TCL_INCLUDE_SPEC) $(CPPFLAGS)
+override CPPFLAGS := -I. -I$(srcdir) $(CPPFLAGS) $(TCL_INCLUDE_SPEC)
 
 # On Windows, we don't link directly with the Tcl library; see below
 ifneq ($(PORTNAME), win32)