don't include system headers in dependency files

System library headers are not strictly part of our build. If they are
changing beneath our feet, then we probably have bigger problems.

Signed-off-by: Brian Norris <computersforpeace@gmail.com>
Suggested-by: Mike Frysinger <vapier@gentoo.org>
Acked-by: Mike Frysinger <vapier@gentoo.org>
Signed-off-by: Brian Norris <computersforpeace@gmail.com>
diff --git a/common.mk b/common.mk
index 7c09ab0..6bfe8de 100644
--- a/common.mk
+++ b/common.mk
@@ -80,7 +80,7 @@
 	$(Q)mkdir -p $(dir $@)
 endif
 	$(call BECHO,CC)
-	$(Q)$(CC) $(CPPFLAGS) $(CFLAGS) -c -o $@ $< -g -MD -MF $(BUILDDIR)/.$(<F).dep
+	$(Q)$(CC) $(CPPFLAGS) $(CFLAGS) -c -o $@ $< -g -MMD -MF $(BUILDDIR)/.$(<F).dep
 
 .SUFFIXES: