Makefile: fix default regex settings on Darwin

Makefile: fix default regex settings on Darwin

t0070-fundamental.sh fails on Mac OS X 10.8:

	$ uname -a
	Darwin lustrous 12.2.0 Darwin Kernel Version 12.2.0:
	Sat Aug 25 00:48:52 PDT 2012;
	root:xnu-2050.18.24~1/RELEASE_X86_64 x86_64

	$ ./t0070-fundamental.sh -v
	fatal: regex bug confirmed: re-build git with NO_REGEX=1

Fix it by using Git's regex library.

Reviewed-by: Jonathan Nieder <redacted>
Signed-off-by: David Aguilar <redacted>
Signed-off-by: Junio C Hamano <redacted>