Add option to use pkg-config rather than pg_config

We shouldn't be using pg_config to get build options for FS from
libpq.  pg_config just tells us what was used to build postgresql, not
what we should use.

See:
http://bugs.debian.org/cgi-bin/bugreport.cgi?bug=725007

Make this optional for now until we're comfortable it works
everywhere.

FS-5821 --resolve
Use pkg-config for libpq by default

FS-5821 --resolve