Stop using compat/regex.c on platforms with working regexp library

Stop using compat/regex.c on platforms with working regexp library

We used to have non-POSIX comformant BRE in our code, and linked with GNU
regexp library on a few platforms (Darwin, FreeBSD and AIX) to work it
around.  This was backwards.

We've fixed the broken regexps to use ERE that native regexp libraries on
these platforms can handle just fine.  There is no need to link with GNU
regexp library on these platforms anymore.

Tested-on-AIX-by: Mike Ralphson <mike@abacus.co.uk>
Tested-on-FreeBSD-by: Jeff King <peff@peff.net>
Tested-on-Darwin-by: Arjen Laarhoven <arjen@yaph.org>
Tested-on-Darwin-by: Pieter de Bie <pieter@frim.nl>
Signed-off-by: Junio C Hamano <gitster@pobox.com>