Put back parallel-safety guards in plpython and src/test/regress/.

I'd hoped that commit 3b8f6e75f was sufficient to ensure parallel safety
even when a build started in a subdirectory requires rebuilding of
generated headers.  This isn't so, because making submake-generated-headers
a prerequisite of "all" isn't enough to ensure it's completed before
starting on "all"'s other prerequisites.  The explicit dependencies we put
on the recursive make targets ensure safe ordering before we recurse into
child directories, but they don't protect targets to be made in the current
directory.  Hence, put back some ordering dependencies in directories that
we've traditionally expected to be starting points for "standalone" builds,
to wit src/pl/plpython and src/test/regress.  (The former needs this in
order to minimize the work involved in building for both python 2 and
python 3; the latter to support packagings that make the regression tests
available for out-of-build-tree execution.)  Adjust some other dependencies
so that these two cases work correctly even at high -j settings.

I'm not terribly happy with this partial solution, but I don't see a
way to do better without massive makefile restructuring, which we surely
aren't doing at this point in the development cycle.  In any case, it's
little if any worse than what we had in prior releases.

Discussion: https://postgr.es/m/1523353963.8169.26.camel@gunduz.org

src/Makefile.global.in

@@ -563,11 +563,11 @@ endif
 submake-libpq:
 	$(MAKE) -C $(libpq_builddir) all
 
-submake-libpgport:
+submake-libpgport: | submake-generated-headers
 	$(MAKE) -C $(top_builddir)/src/port all
 	$(MAKE) -C $(top_builddir)/src/common all
 
-submake-libpgfeutils:
+submake-libpgfeutils: | submake-generated-headers
 	$(MAKE) -C $(top_builddir)/src/port all
 	$(MAKE) -C $(top_builddir)/src/common all
 	$(MAKE) -C $(top_builddir)/src/fe_utils all

src/pl/plpython/Makefile

@@ -99,6 +99,9 @@ include $(top_srcdir)/src/Makefile.shlib
 
 all: all-lib
 
+# Ensure parallel safety if a build is started in this directory
+$(OBJS): | submake-generated-headers
+
 install: all install-lib install-data
 
 installdirs: installdirs-lib

src/test/regress/GNUmakefile

@@ -43,7 +43,8 @@ pg_regress$(X): pg_regress.o pg_regress_main.o $(WIN32RES) | submake-libpgport
 pg_regress.o: pg_regress.c $(top_builddir)/src/port/pg_config_paths.h
 pg_regress.o: override CPPFLAGS += -I$(top_builddir)/src/port $(EXTRADEFS)
 
-$(top_builddir)/src/port/pg_config_paths.h: $(top_builddir)/src/Makefile.global
+# note: because of the submake dependency, this rule's action is really a no-op
+$(top_builddir)/src/port/pg_config_paths.h: | submake-libpgport
 	$(MAKE) -C $(top_builddir)/src/port pg_config_paths.h
 
 install: all installdirs
@@ -65,6 +66,9 @@ include $(top_srcdir)/src/Makefile.shlib
 
 all: all-lib
 
+# Ensure parallel safety if a build is started in this directory
+$(OBJS): | submake-libpgport submake-generated-headers
+
 # Test input and expected files.  These are created by pg_regress itself, so we
 # don't have a rule to create them.  We do need rules to clean them however.
 input_files = $(patsubst $(srcdir)/input/%.source,sql/%.sql, $(wildcard $(srcdir)/input/*.source))
@@ -105,7 +109,7 @@ $(top_builddir)/contrib/spi/refint$(DLSUFFIX): | submake-contrib-spi ;
 
 $(top_builddir)/contrib/spi/autoinc$(DLSUFFIX): | submake-contrib-spi ;
 
-submake-contrib-spi: | submake-libpgport
+submake-contrib-spi: | submake-libpgport submake-generated-headers
 	$(MAKE) -C $(top_builddir)/contrib/spi
 
 .PHONY: submake-contrib-spi