Fix for using Makefile.custom

Suggested/Submitted by: darcy@druid.druid.com
Cleaner Method Submitted by: Bryan Henderson <bryanh@giraffe.netgate.net>

src/Makefile.global

@@ -7,7 +7,7 @@
 #
 #
 # IDENTIFICATION
-#    $Header: /cvsroot/pgsql/src/Attic/Makefile.global,v 1.33 1996/09/23 08:37:41 scrappy Exp $
+#    $Header: /cvsroot/pgsql/src/Attic/Makefile.global,v 1.34 1996/09/23 08:43:40 scrappy Exp $
 #
 # NOTES
 #    This is seen by any Makefiles that include mk/postgres.mk. To
@@ -322,6 +322,7 @@ objdir=		obj
 # when you upgrade the source) so you have to create at least an
 # empty one.  Run "touch Makefile.custom" in the source directory
 # to create an empty customization file.  Make your local changes.
-
-# include $(MKDIR)/../Makefile.custom
+ifneq ($(wildcard $(MKDIR)/../Makefile.custom), )
+include $(MKDIR)/../Makefile.custom
+endif