Build src/ before contrib/ in make world

This fixes failures under parallel make when contrib modules use a
generated backend header file (such as errcodes.h).

GNUmakefile.in

@@ -20,6 +20,9 @@ $(call recurse,world,doc src config contrib,all)
 world:
 	+@echo "PostgreSQL, contrib, and documentation successfully made. Ready to install."
 
+# build src/ before contrib/
+world-contrib-recurse: world-src-recurse
+
 html man:
 	$(MAKE) -C doc $@
 
@@ -33,6 +36,9 @@ $(call recurse,install-world,doc src config contrib,install)
 install-world:
 	+@echo "PostgreSQL, contrib, and documentation installation complete."
 
+# build src/ before contrib/
+install-world-contrib-recurse: install-world-src-recurse
+
 $(call recurse,installdirs uninstall coverage,doc src config)
 
 $(call recurse,distprep,doc src config contrib)