ecpg: Don't link compatlib with libpq

It doesn't actually use libpq.  But we need to keep libpq in the
CPPFLAGS for building, because compatlib uses ecpglib.h which uses
libpq-fe.h, but we don't need to refer to libpq for linking.

reviewed by Tom Lane
Add pkg-config files for libpq and ecpg libraries

This will hopefully be easier to use than pg_config for users who are
already used to the pkg-config interface.  It also works better for
multi-arch installations.

reviewed by Tom Lane