hstore_plperl: Move port-specific parts to later in the makefile

PORTNAME isn't set until the global makefiles have been included.

contrib/hstore_plperl/Makefile

@@ -5,11 +5,6 @@ OBJS = hstore_plperl.o
 
 PG_CPPFLAGS = -I$(top_srcdir)/src/pl/plperl -I$(top_srcdir)/contrib/hstore -I$(perl_archlibexp)/CORE
 
-# see plperl
-ifeq ($(PORTNAME), win32)
-PG_CPPFLAGS += -DPLPERL_HAVE_UID_GID -Wno-comment
-endif
-
 EXTENSION = hstore_plperl hstore_plperlu
 DATA = hstore_plperl--1.0.sql hstore_plperlu--1.0.sql
 
@@ -26,3 +21,8 @@ top_builddir = ../..
 include $(top_builddir)/src/Makefile.global
 include $(top_srcdir)/contrib/contrib-global.mk
 endif
+
+# see plperl
+ifeq ($(PORTNAME), win32)
+override CPPFLAGS += -DPLPERL_HAVE_UID_GID -Wno-comment
+endif