Link libpq with libdl if the platform needs that.

Link libpq with libdl if the platform needs that.

Since b0635bfda, libpq uses dlopen() and related functions.  On some
platforms these are not supplied by libc, but by a separate library
libdl, in which case we need to make sure that that dependency is
known to the linker.  Meson seems to take care of that automatically,
but the Makefile didn't cater for it.

Author: Tom Lane <tgl@sss.pgh.pa.us>
Discussion: https://postgr.es/m/1328170.1752082586@sss.pgh.pa.us
Backpatch-through: 18