Additional attempt at bug #40.

If libpq isn't already in the system shared library search
path directories then it might not be found when configure
tries to run the test program.

Build the path to libpq into the test executable via linker
flags.  The linker flags for this vary based on platform.
The logic was based on logic in postgresql/src/makefiles
in the Postgresql source tree.

Also perform a runtime check to make sure the runtime
libpq is thread-safe.
(cherry picked from commit 2ee266a20be37964b6aa0df1554a0b5998fbba4a)

Branch
------
master

Details
-------
http://git.postgresql.org/gitweb?p=slony1-engine.git;a=commitdiff;h=81b040b7151a98fbfd19e8e68c1e3dec00d6cbf8

Modified Files
--------------
config/acx_libpq.m4 |   30 ++++++++++++++++++++++++++++--
src/slon/slon.c     |    6 ++++++
2 files changed, 34 insertions(+), 2 deletions(-)