Allow ecpg to be built stand-alone, allow parallel libpq make

This change defines SHLIB_PREREQS for the libpgport dependency, rather
than using a makefile rule.  This was broken in PG 12.

Reported-by: Filip Janus

Discussion: https://postgr.es/m/E5Dc85EGUY4wyG8cjAU0qoEdCJxGK_qhW1s9qSuYq9A@mail.gmail.com

Author: Dagfinn Ilmari Mannsåker (for libpq)

Backpatch-through: 12

Author: bmomjian

src/interfaces/ecpg/pgtypeslib/Makefile

@@ -24,6 +24,7 @@ override CFLAGS += $(PTHREAD_CFLAGS)
 
 SHLIB_LINK_INTERNAL = -lpgcommon_shlib -lpgport_shlib
 SHLIB_LINK += $(filter -lintl -lm, $(LIBS))
+SHLIB_PREREQS = submake-libpgport
 
 SHLIB_EXPORTS = exports.txt
 

src/interfaces/libpq/Makefile

@@ -85,13 +85,12 @@ endif
 ifeq ($(PORTNAME), win32)
 SHLIB_LINK += -lshell32 -lws2_32 -lsecur32 $(filter -leay32 -lssleay32 -lcomerr32 -lkrb5_32, $(LIBS))
 endif
+SHLIB_PREREQS = submake-libpgport
 
 SHLIB_EXPORTS = exports.txt
 
 all: all-lib
 
-all-lib: | submake-libpgport
-
 # Shared library stuff
 include $(top_srcdir)/src/Makefile.shlib
 backend_src = $(top_srcdir)/src/backend