When building shared libraries on HPUX with gcc, forcibly include -lgcc

to ensure any needed compiler support routines are included.  This is
arguably appropriate on *every* gcc platform, but for the moment I'll take
the conservative approach of only doing it on a platform where it's
provably useful.  Per complaint from Heiko Lehmann, 13-Feb-03, as well
as personal experience --- contrib/pgstattuple has never worked for me,
but it does now.