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

Back-patch commit dddfc4cb2, which broke LDFLAGS and related Makefile
variables into two parts, one for within-build-tree library references and
one for external libraries, to ensure that the order of -L flags has all
of the former before all of the latter.  This turns out to fix a problem
recently noted on buildfarm member peripatus, that we attempted to
incorporate code from libpgport.a into a shared library.  That will fail on
platforms that are sticky about putting non-PIC code into shared libraries.
(It's quite surprising we hadn't seen such failures before, since the code
in question has been like that for a long time.)

I think that peripatus' problem could have been fixed with just a subset
of this patch; but since the previous issue of accidentally linking to the
wrong copy of a Postgres shlib seems likely to bite people in the field,
let's just back-patch the whole change.  Now that commit dddfc4cb2 has
survived some beta testing, I'm less afraid to back-patch it than I was
at the time.

This also fixes undesired inclusion of "-DFRONTEND" in pg_config's CPPFLAGS
output (in 9.6 and up) and undesired inclusion of "-L../../src/common" in
its LDFLAGS output (in all supported branches).

Back-patch to v10 and older branches; this is already in v11.

Discussion: https://postgr.es/m/20180704234304.bq2dxispefl65odz@ler-imac.local

contrib/dblink/Makefile

@@ -3,7 +3,7 @@
 MODULE_big = dblink
 OBJS	= dblink.o $(WIN32RES)
 PG_CPPFLAGS = -I$(libpq_srcdir)
-SHLIB_LINK = $(libpq)
+SHLIB_LINK_INTERNAL = $(libpq)
 
 EXTENSION = dblink
 DATA = dblink--1.2.sql dblink--1.1--1.2.sql dblink--1.0--1.1.sql \

contrib/hstore_plperl/Makefile

@@ -28,7 +28,7 @@ ifeq ($(PORTNAME), win32)
 # these settings are the same as for plperl
 override CPPFLAGS += -DPLPERL_HAVE_UID_GID -Wno-comment
 # ... see silliness in plperl Makefile ...
-SHLIB_LINK += $(sort $(wildcard ../../src/pl/plperl/libperl*.a))
+SHLIB_LINK_INTERNAL += $(sort $(wildcard ../../src/pl/plperl/libperl*.a))
 else
 rpathdir = $(perl_archlibexp)/CORE
 SHLIB_LINK += $(perl_embed_ldflags)

contrib/hstore_plpython/Makefile

@@ -26,7 +26,7 @@ endif
 # We must link libpython explicitly
 ifeq ($(PORTNAME), win32)
 # ... see silliness in plpython Makefile ...
-SHLIB_LINK += $(sort $(wildcard ../../src/pl/plpython/libpython*.a))
+SHLIB_LINK_INTERNAL += $(sort $(wildcard ../../src/pl/plpython/libpython*.a))
 else
 rpathdir = $(python_libdir)
 SHLIB_LINK += $(python_libspec) $(python_additional_libs)

contrib/ltree_plpython/Makefile

@@ -26,7 +26,7 @@ endif
 # We must link libpython explicitly
 ifeq ($(PORTNAME), win32)
 # ... see silliness in plpython Makefile ...
-SHLIB_LINK += $(sort $(wildcard ../../src/pl/plpython/libpython*.a))
+SHLIB_LINK_INTERNAL += $(sort $(wildcard ../../src/pl/plpython/libpython*.a))
 else
 rpathdir = $(python_libdir)
 SHLIB_LINK += $(python_libspec) $(python_additional_libs)

contrib/oid2name/Makefile

@@ -7,7 +7,7 @@ PROGRAM = oid2name
 OBJS	= oid2name.o $(WIN32RES)
 
 PG_CPPFLAGS = -I$(libpq_srcdir)
-PG_LIBS = $(libpq_pgport)
+PG_LIBS_INTERNAL = $(libpq_pgport)
 
 ifdef USE_PGXS
 PG_CONFIG = pg_config

contrib/postgres_fdw/Makefile

@@ -5,7 +5,7 @@ OBJS = postgres_fdw.o option.o deparse.o connection.o shippable.o $(WIN32RES)
 PGFILEDESC = "postgres_fdw - foreign data wrapper for PostgreSQL"
 
 PG_CPPFLAGS = -I$(libpq_srcdir)
-SHLIB_LINK = $(libpq)
+SHLIB_LINK_INTERNAL = $(libpq)
 
 EXTENSION = postgres_fdw
 DATA = postgres_fdw--1.0.sql

contrib/spi/Makefile

@@ -17,8 +17,6 @@ DOCS = $(addsuffix .example, $(MODULES))
 # comment out if you want a quieter refint package for other uses
 PG_CPPFLAGS = -DREFINT_VERBOSE
 
-LDFLAGS_SL += -L$(top_builddir)/src/port -lpgport
-
 ifdef USE_PGXS
 PG_CONFIG = pg_config
 PGXS := $(shell $(PG_CONFIG) --pgxs)

contrib/vacuumlo/Makefile

@@ -7,7 +7,7 @@ PROGRAM = vacuumlo
 OBJS	= vacuumlo.o $(WIN32RES)
 
 PG_CPPFLAGS = -I$(libpq_srcdir)
-PG_LIBS = $(libpq_pgport)
+PG_LIBS_INTERNAL = $(libpq_pgport)
 
 ifdef USE_PGXS
 PG_CONFIG = pg_config