FS-5512 --resolve ; clean up PostgreSQL libpq client detection in configure

FS-5512 --resolve ; clean up PostgreSQL libpq client detection in configure

Clean up the libpq autodection and make sure it functionswith both pg_config and pkg-config.

if you are installing into a non-FHS standard location such as what happens with RPMs from the Postgres yum repo, you will need to make sure pkg-config can fild the libpq.pc file.
example: export PKG_CONFIG_PATH=/usr/pgsql-9.3/lib/pkgconfig

you may also want to make sure your path can find pg_config
example: export PATH=/usr/pgsql-9.3/bin:$PATH