patch to fix client only builds

I'm trying to do client only builds on a bunch of legacy platforms and
noticed that the include path is messed up...if keywords.o is not
already built, it fails to build be because src/backend/parser but not
src/backend is in the include path. (keywords.c includes
parser/gram.h).

The following fixes it.  Probably not the right thing exactly but it works:

Index: Makefile
===================================================================
RCS file: /projects/cvsroot/pgsql/src/backend/parser/Makefile,v
retrieving revision 1.48
diff -r1.48 Makefile
13c13
< override CPPFLAGS := -I$(srcdir) $(CPPFLAGS)
---
> override CPPFLAGS := -I$(subdir) -I.. $(CPPFLAGS)

This would be a nice backpatch to 8.3 (and possibly earlier, I didn't check).

merlin