Only include the libpq header directory OR the server header directory but not both

This addresses bug 315 where a debian install had libpq-dev 9.3 installed
and postgresql-server-dev 8.4 installed.  The libpq headers are in /usr/include/postgresql
but the server headers are in /usr/lib/postgresql/8.4/server.

configure checks were failing because we were pulling in a mixutre of headers
from both versions.  The only slony component that needs the server
headers are the backend functions and they shouldn't need to use libpq headers.

This patch sets CPPFLAGS as approriate for one or the other but not both

Conflicts:
	Makefile.global.in
	RELEASE

Branch
------
REL_2_0_STABLE

Details
-------
http://git.postgresql.org/gitweb?p=slony1-engine.git;a=commitdiff;h=f3ae6a62acb3ba37ba0b54105c09ea7acdcd3121

Modified Files
--------------
Makefile.global.in            |    5 ++++-
RELEASE                       |   10 ++++++++++
config/acx_libpq.m4           |   10 ++++++----
configure.ac                  |    3 ++-
src/backend/Makefile          |    2 +-
src/parsestatements/Makefile  |    2 ++
src/slon/Makefile             |    2 +-
src/slonik/Makefile           |    1 +
src/slony_logshipper/Makefile |    2 +-
9 files changed, 28 insertions(+), 9 deletions(-)