Use -isystem for LLVM include directories

LLVM's headers produce warnings under some of the warning options we
might like to use.  For example, -Wshadow=local reports several
warnings from LLVM's headers when compiling llvmjit_inline.cpp.  To
work around that, add the include directories reported by llvm-config
with -isystem rather than -I, which makes the compiler treat them as
system headers and not report warnings from them.  In meson, this is a
built-in facility of the dependency() function, in configure we
implement it ourselves.

An alternative solution would have been to use "#pragma GCC
system_header", as is already done elsewhere in the tree.  But that
seems less elegant here.  Either, we would have to potentially create
a separate wrapper for each LLVM header, or we would have to route all
LLVM includes through a common header, which would break modularity.

Reviewed-by: Peter Geoghegan <pg@bowt.ie>
Reviewed-by: Chao Li <li.evan.chao@gmail.com>
Discussion: https://www.postgresql.org/message-id/flat/84b6f128-91f5-480e-8a9e-7d0e8f538cea%40eisentraut.org

config/llvm.m4

@@ -46,10 +46,13 @@ AC_DEFUN([PGAC_LLVM_SUPPORT],
   # clear what the minimum version is.
 
   # Collect compiler flags necessary to build the LLVM dependent
-  # shared library.
+  # shared library.  The include directories are added with -isystem
+  # rather than -I, so that warnings from LLVM's own headers are not
+  # reported under the warning options we select for our own code.
   for pgac_option in `$LLVM_CONFIG --cppflags`; do
     case $pgac_option in
-      -I*|-D*) LLVM_CPPFLAGS="$pgac_option $LLVM_CPPFLAGS";;
+      -I*) LLVM_CPPFLAGS="-isystem ${pgac_option#-I} $LLVM_CPPFLAGS";;
+      -D*) LLVM_CPPFLAGS="$pgac_option $LLVM_CPPFLAGS";;
     esac
   done
 

configure

@@ -5105,10 +5105,13 @@ fi
   # clear what the minimum version is.
 
   # Collect compiler flags necessary to build the LLVM dependent
-  # shared library.
+  # shared library.  The include directories are added with -isystem
+  # rather than -I, so that warnings from LLVM's own headers are not
+  # reported under the warning options we select for our own code.
   for pgac_option in `$LLVM_CONFIG --cppflags`; do
     case $pgac_option in
-      -I*|-D*) LLVM_CPPFLAGS="$pgac_option $LLVM_CPPFLAGS";;
+      -I*) LLVM_CPPFLAGS="-isystem ${pgac_option#-I} $LLVM_CPPFLAGS";;
+      -D*) LLVM_CPPFLAGS="$pgac_option $LLVM_CPPFLAGS";;
     esac
   done
 

meson.build

@@ -924,7 +924,10 @@ endif
 llvmopt = get_option('llvm')
 llvm = not_found_dep
 if have_cxx
-  llvm = dependency('llvm', version: '>=14', method: 'config-tool', required: llvmopt)
+  # Use include_type 'system' so that warnings from LLVM's own headers are
+  # not reported under the warning options we select for our own code.
+  llvm = dependency('llvm', version: '>=14', method: 'config-tool',
+                    include_type: 'system', required: llvmopt)
 
   if llvm.found()