pkg-config files for libpq and ecpg

I'll take another stab at providing pkg-config files for the client-side
libraries.

The main reason this time around is that this works a lot better (or at
all) for multi-arch library installations.

Another is that pkg-config has become a lot smarter and flexible over
the years, and it's probably a better choice for users who are already
used to its interface. There is a lot of confusion, for example, about
what pg_config --libs really means. We often evade that by saying,
well, those are the libraries we linked with, but there is a lack of
clarity in that context about what libraries a user should link with.

The way it's implemented, it doesn't require manual maintenance, so it
should not be much of a bother.

A side issue that arose: libecpg_compat is linked with libpq, but
doesn't seem to use it. This was added many years ago in
cd75f94dafd43358305811b7576ad75d889097e3, but it doesn't appear to be
required anymore. Needs some checking.