Usage of internal libpq distribution is broken

[siddharthab]

When libpq is not installed in the user's system, the user does not get an appropriate error message. The package configure script attempts to use a bundled distribution but fails because:

1. When PG_INCDIR is not found, it is set to src/libpq instead of libpq, which makes the compilation fail because of missing headers.
2. PG_LIBDIR is never set if not found externally, so the compilation fails at the linking step.

I think it's fine to not be able to use a bundled distribution, but I think the user should be messaged accordingly.