On OS X, allow Configure to override $ld with -Dld=...

Since it was added, hints/darwin.sh had been ignoring any user configuration
options and unconditionally setting ld='cc'. Instead, it should take a user
supplied value if one is given, otherwise defaulting to 'cc'.

The strange thing is that this was never noticed when commits cb3fc4263509f28c
and 69625aa92a91bf4c (May 2003) which added code which was intended offer
alternative behaviour if ld contained MACOSX_DEVELOPMENT_TARGET.