config.mak.uname: use clang for Mac OS X 10.6

Gcc under Mac OX 10.6 throws an internal compiler error:

CC combine-diff.o
    combine-diff.c: In function ‘diff_tree_combined’:
    combine-diff.c:1391: internal compiler error: Segmentation fault

while attempting to build Git at 5b442c4f (tree-diff: catch integer
overflow in combine_diff_path allocation, 2016-02-19).

As clang that ships with the version does not have the same bug,
make Git compile under Mac OS X 10.6 by using clang instead of gcc
to work this around, as it is unlikely that we will see fixed GCC
on that platform.

Later versions of Mac OSX/Xcode only provide clang, and gcc is a
wrapper to it.

Signed-off-by: Torsten Bögershausen <tboegi@web.de>
Signed-off-by: Junio C Hamano <gitster@pobox.com>