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>

config.mak.uname

@@ -102,6 +102,9 @@ ifeq ($(uname_S),Darwin)
 	ifeq ($(shell expr "$(uname_R)" : '[15]\.'),2)
 		NO_STRLCPY = YesPlease
 	endif
+	ifeq ($(shell test "`expr "$(uname_R)" : '\([0-9][0-9]*\)\.'`" -eq 10 && echo 1),1)
+		CC = clang
+	endif
 	ifeq ($(shell test "`expr "$(uname_R)" : '\([0-9][0-9]*\)\.'`" -ge 11 && echo 1),1)
 		HAVE_GETDELIM = YesPlease
 	endif