Fix pkg-config files for static linking

Since ea53100d5 (PostgreSQL 12), the shipped pkg-config files have
been broken for statically linking libpq because libpgcommon and
libpgport are missing.  This patch adds those two missing private
dependencies (in a non-hardcoded way).

Reported-by: Filip Gospodinov <f@gospodinov.ch>
Discussion: https://www.postgresql.org/message-id/flat/c7108bde-e051-11d5-a234-99beec01ce2a@gospodinov.ch

src/Makefile.shlib

@@ -400,7 +400,7 @@ endif # PORTNAME == cygwin || PORTNAME == win32
 # those that point inside the build or source tree.  Use sort to
 # remove duplicates.  Also record the -l flags necessary for static
 # linking, but not those already covered by Requires.private.
-	echo 'Libs.private: $(sort $(filter-out -L.% -L$(top_srcdir)/%,$(filter -L%,$(LDFLAGS) $(SHLIB_LINK)))) $(filter-out $(PKG_CONFIG_REQUIRES_PRIVATE:lib%=-l%),$(filter -l%,$(SHLIB_LINK)))' >>$@
+	echo 'Libs.private: $(sort $(filter-out -L.% -L$(top_srcdir)/%,$(filter -L%,$(LDFLAGS) $(SHLIB_LINK)))) $(filter-out $(PKG_CONFIG_REQUIRES_PRIVATE:lib%=-l%),$(filter -l%,$(SHLIB_LINK_INTERNAL) $(SHLIB_LINK)))' >>$@
 
 
 ##