Revert "ecpg: Don't link compatlib with libpq"

This reverts commit 3780fc679cc428c1f211e1728c4281ca15e9746b.

HP-UX didn't like it.  There would probably be a way to fix that, but
since the net effect of all of this is zero because ecpg ends up using
libpq anyway, it's not worth bothering further.