Makefile: make gcov invocation configurable

If you customize CC to use a different version of gcc, most likely you
also need to use a different version of gcov.  Make it configurable.

Signed-off-by: Thomas Rast <trast@student.ethz.ch>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

Makefile

@@ -308,6 +308,7 @@ TCL_PATH = tclsh
 TCLTK_PATH = wish
 PTHREAD_LIBS = -lpthread
 PTHREAD_CFLAGS =
+GCOV = gcov
 
 export TCL_PATH TCLTK_PATH
 
@@ -2312,7 +2313,7 @@ coverage-build: coverage-clean
 
 coverage-report:
 	$(QUIET_GCOV)for dir in $(object_dirs); do \
-		gcov $(GCOVFLAGS) --object-directory=$$dir $$dir*.c || exit; \
+		$(GCOV) $(GCOVFLAGS) --object-directory=$$dir $$dir*.c || exit; \
 	done
 
 coverage-untested-functions: coverage-report