Fix SHLIB_PREREQS use in contrib, allowing PGXS builds

dblink and postgres_fdw use SHLIB_PREREQS = submake-libpq to build libpq
first.  This doesn't work in a PGXS build, because there is no libpq to
build.  So just omit setting SHLIB_PREREQS in this case.

Note that PGXS users can still use SHLIB_PREREQS (although it is not
documented).  The problem here is only that contrib modules can be built
in-tree or using PGXS, and the prerequisite is only applicable in the
former case.

Commit 6697aa2b previously attempted to
address this by creating a somewhat fake submake-libpq target in
Makefile.global.  That was not the right fix, and it was also done in a
nonportable way, so revert that.

contrib/dblink/Makefile

@@ -4,7 +4,6 @@ MODULE_big = dblink
 OBJS	= dblink.o $(WIN32RES)
 PG_CPPFLAGS = -I$(libpq_srcdir)
 SHLIB_LINK = $(libpq)
-SHLIB_PREREQS = submake-libpq
 
 EXTENSION = dblink
 DATA = dblink--1.1.sql dblink--1.0--1.1.sql dblink--unpackaged--1.0.sql
@@ -19,6 +18,7 @@ PG_CONFIG = pg_config
 PGXS := $(shell $(PG_CONFIG) --pgxs)
 include $(PGXS)
 else
+SHLIB_PREREQS = submake-libpq
 subdir = contrib/dblink
 top_builddir = ../..
 include $(top_builddir)/src/Makefile.global

contrib/postgres_fdw/Makefile

@@ -6,7 +6,6 @@ PGFILEDESC = "postgres_fdw - foreign data wrapper for PostgreSQL"
 
 PG_CPPFLAGS = -I$(libpq_srcdir)
 SHLIB_LINK = $(libpq)
-SHLIB_PREREQS = submake-libpq
 
 EXTENSION = postgres_fdw
 DATA = postgres_fdw--1.0.sql
@@ -18,6 +17,7 @@ PG_CONFIG = pg_config
 PGXS := $(shell $(PG_CONFIG) --pgxs)
 include $(PGXS)
 else
+SHLIB_PREREQS = submake-libpq
 subdir = contrib/postgres_fdw
 top_builddir = ../..
 include $(top_builddir)/src/Makefile.global

src/Makefile.global.in

@@ -458,23 +458,13 @@ ifeq ($(PORTNAME),cygwin)
 libpq_pgport += $(LDAP_LIBS_FE)
 endif
 
-# If PGXS is not defined, build libpq and libpgport dependencies as required.
-# If the build is with PGXS, then these are supposed to be already built and
-# installed, and we just ensure that the expected files exist.
-ifndef PGXS
+
 submake-libpq:
 	$(MAKE) -C $(libpq_builddir) all
-else
-submake-libpq: $(libdir)/libpq.so ;
-endif
 
-ifndef PGXS
 submake-libpgport:
 	$(MAKE) -C $(top_builddir)/src/port all
 	$(MAKE) -C $(top_builddir)/src/common all
-else
-submake-libpgport: $(libdir)/libpgport.a $(libdir)/libpgcommon.a ;
-endif
 
 .PHONY: submake-libpq submake-libpgport