Clarify comment about gcc -MM.

Originally committed as revision 10875 to svn://svn.ffmpeg.org/ffmpeg/trunk

Makefile

@@ -226,7 +226,7 @@ endif
 .depend: $(SRCS) version.h
 	$(CC) -MM $(CFLAGS) $(SDL_CFLAGS) $(filter-out %.h,$^) 1>.depend
 
-# gcc stupidly only outputs the basename of targets
+# gcc stupidly only outputs the basename of targets with -MM
 .vhookdep: $(ALLHOOKS_SRCS)
 	$(CC) $(VHOOKCFLAGS) -MM $^ | sed 's,^\([a-z]\),vhook/\1,' > $@