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

Branch
------
master

Details
-------
http://git.postgresql.org/gitweb?p=slony1-engine.git;a=commitdiff;h=5e1ad5c7731c8daa5ebeac236c9f09f58a385245

Modified Files
--------------
Makefile.global.in            |    3 ++-
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 +-
8 files changed, 16 insertions(+), 9 deletions(-)