Makefile: add CC to TRACK_CFLAGS

Change the git make process so that everything's rebuilt if the CC is
changed. Before we wouldn't rebuilt if e.g. the CC variable was
changed from gcc to clang.

Signed-off-by: Ævar Arnfjörð Bjarmason <avarab@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

Makefile

@@ -1974,7 +1974,7 @@ cscope:
 	$(FIND) . -name '*.[hcS]' -print | xargs cscope -b
 
 ### Detect prefix changes
-TRACK_CFLAGS = $(subst ','\'',$(ALL_CFLAGS)):\
+TRACK_CFLAGS = $(CC):$(subst ','\'',$(ALL_CFLAGS)):\
              $(bindir_SQ):$(gitexecdir_SQ):$(template_dir_SQ):$(prefix_SQ)
 
 GIT-CFLAGS: FORCE