Now that st_add4() has been patched to work around the gcc 4.2.x
compiler crash, revert the sledge-hammer approach of forcing Mac OS X
10.6 to unconditionally use 'clang' rather than the default compiler
(gcc).
Signed-off-by: Eric Sunshine <sunshine@sunshineco.com> Signed-off-by: Junio C Hamano <gitster@pobox.com>