Prevent accidental linking of system-supplied copies of libpq.so etc.

We were being careless in some places about the order of -L switches in
link command lines, such that -L switches referring to external directories
could come before those referring to directories within the build tree.
This made it possible to accidentally link a system-supplied library, for
example /usr/lib/libpq.so, in place of the one built in the build tree.
Hilarity ensued, the more so the older the system-supplied library is.

To fix, break LDFLAGS into two parts, a sub-variable LDFLAGS_INTERNAL
and the main LDFLAGS variable, both of which are "recursively expanded"
so that they can be incrementally adjusted by different makefiles.
Establish a policy that -L switches for directories in the build tree
must always be added to LDFLAGS_INTERNAL, while -L switches for external
directories must always be added to LDFLAGS.  This is sufficient to
ensure a safe search order.  For simplicity, we typically also put -l
switches for the respective libraries into those same variables.
(Traditional make usage would have us put -l switches into LIBS, but
cleaning that up is a project for another day, as there's no clear
need for it.)

This turns out to also require separating SHLIB_LINK into two variables,
SHLIB_LINK and SHLIB_LINK_INTERNAL, with a similar rule about which
switches go into which variable.  And likewise for PG_LIBS.

Although this change might appear to affect external users of pgxs.mk,
I think it doesn't; they shouldn't have any need to touch the _INTERNAL
variables.

In passing, tweak src/common/Makefile so that the value of CPPFLAGS
recorded in pg_config lacks "-DFRONTEND" and the recorded value of
LDFLAGS lacks "-L../../../src/common".  Both of those things are
mistakes, apparently introduced during prior code rearrangements,
as old versions of pg_config don't print them.  In general we don't
want anything that's specific to the src/common subdirectory to
appear in those outputs.

This is certainly a bug fix, but in view of the lack of field
complaints, I'm unsure whether it's worth the risk of back-patching.
In any case it seems wise to see what the buildfarm makes of it first.

Discussion: https://postgr.es/m/25214.1522604295@sss.pgh.pa.us

src/interfaces/ecpg/compatlib/Makefile

@@ -22,8 +22,8 @@ override CPPFLAGS := -I../include -I$(top_srcdir)/src/interfaces/ecpg/include \
 	-I$(libpq_srcdir) -DFRONTEND $(CPPFLAGS)
 override CFLAGS += $(PTHREAD_CFLAGS)
 
-SHLIB_LINK = -L../ecpglib -lecpg -L../pgtypeslib -lpgtypes $(libpq) \
-	$(filter -lintl -lm, $(LIBS)) $(PTHREAD_LIBS)
+SHLIB_LINK_INTERNAL = -L../ecpglib -lecpg -L../pgtypeslib -lpgtypes $(libpq)
+SHLIB_LINK = $(filter -lintl -lm, $(LIBS)) $(PTHREAD_LIBS)
 SHLIB_PREREQS = submake-ecpglib submake-pgtypeslib
 
 SHLIB_EXPORTS = exports.txt

src/interfaces/ecpg/ecpglib/Makefile

@@ -35,7 +35,8 @@ ifneq ($(PORTNAME), win32)
 OBJS += thread.o
 endif
 
-SHLIB_LINK = -L../pgtypeslib -lpgtypes $(libpq) $(filter -lintl -lm, $(LIBS)) $(PTHREAD_LIBS)
+SHLIB_LINK_INTERNAL = -L../pgtypeslib -lpgtypes $(libpq)
+SHLIB_LINK = $(filter -lintl -lm, $(LIBS)) $(PTHREAD_LIBS)
 SHLIB_PREREQS = submake-libpq submake-pgtypeslib
 
 SHLIB_EXPORTS = exports.txt

src/interfaces/ecpg/pgtypeslib/Makefile

@@ -25,7 +25,7 @@ override CFLAGS += $(PTHREAD_CFLAGS)
 # Need to recompile any libpgport object files
 LIBS := $(filter-out -lpgport, $(LIBS))
 
-SHLIB_LINK += -lm
+SHLIB_LINK += $(filter -lm, $(LIBS))
 
 SHLIB_EXPORTS = exports.txt
 

src/interfaces/ecpg/test/Makefile.regress

@@ -5,8 +5,9 @@ override CPPFLAGS := -I../../include -I$(top_srcdir)/src/interfaces/ecpg/include
 	-I$(libpq_srcdir) $(CPPFLAGS)
 override CFLAGS += $(PTHREAD_CFLAGS)
 
-override LDFLAGS := -L../../ecpglib -L../../pgtypeslib $(filter-out -l%, $(libpq)) $(LDFLAGS)
-override LIBS := -lecpg -lpgtypes $(filter -l%, $(libpq)) $(LIBS) $(PTHREAD_LIBS)
+LDFLAGS_INTERNAL += -L../../ecpglib -lecpg -L../../pgtypeslib -lpgtypes $(libpq)
+
+override LIBS += $(PTHREAD_LIBS)
 
 # Standard way to invoke the ecpg preprocessor
 ECPG = ../../preproc/ecpg --regression -I$(srcdir)/../../include -I$(srcdir)

src/interfaces/ecpg/test/compat_informix/Makefile

@@ -6,8 +6,7 @@ include $(top_srcdir)/$(subdir)/../Makefile.regress
 # Use special informix compatibility switch for all tests in this directory
 ECPG += -C INFORMIX
 
-override LDFLAGS := -L../../compatlib $(LDFLAGS)
-override LIBS := -lecpg_compat $(LIBS)
+LDFLAGS_INTERNAL += -L../../compatlib -lecpg_compat
 
 TESTS = test_informix test_informix.c \
         test_informix2 test_informix2.c \

src/interfaces/libpq/test/Makefile

@@ -3,11 +3,11 @@ top_builddir = ../../../..
 include $(top_builddir)/src/Makefile.global
 
 ifeq ($(PORTNAME), win32)
-LDLIBS += -lws2_32
+LDFLAGS += -lws2_32
 endif
 
 override CPPFLAGS := -I$(libpq_srcdir) $(CPPFLAGS)
-override LDLIBS := $(libpq_pgport) $(LDLIBS)
+LDFLAGS_INTERNAL += $(libpq_pgport)
 
 PROGS = uri-regress