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

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