Cygwin v1.7 uses the regex library from newlib which does not pass git's
tests, so don't use it. This fixes failures in t4018 and t4034.
Continue to use the platform supplied regex library for earlier versions.
Signed-off-by: Mark Levedahl <mlevedahl@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>