Kludge to remove -lpam from LIBS in Makefiles.

Per Issue #7 from armanddp, it seems the RHEL/CentOS packages
of Postgres are built against libpam, which shows up in
the LIBS reported by pg_config. We don't actually need to
link in libpam for pg_repack, so manually remove it from
our LIBS. This helps a little bit for building on
RHEL/CentOS, though the issue of libpgport.a missing
on that platform still looms.