make: Remove -pthread on Darwin (it is included by cstdlib).

make: Remove -pthread on Darwin (it is included by cstdlib).

As discussed in

http://lists.apple.com/archives/Unix-porting/2005/Mar/msg00019.html

the Mac OS X C standard library is always thread safe and always
includes the pthread library. So explicitly using -pthread causes an
'unrecognized option' compiler warning.

This patch clears PTHREAD_LIBS if Darwin is detected.

Signed-off-by: Ted Pavlic <ted@tedpavlic.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>