Exclude system headers from .d files

We currently generated .d dependency files with the -MD option to cc.  That
includes system header files in the dependencies, which isn't often useful
and makes the .d more complicated than necessary.

This changes to -MMD which excludes system headers.
Signed-off-by: David Gibson <david@gibson.dropbear.id.au>

Makefile-ccan

@@ -135,7 +135,7 @@ MODS:=$(MODS_WITH_SRC) $(MODS_NO_SRC)
 default: libccan.a
 
 # Automatic dependency generation: makes ccan/*/*.d files.
-DEPGEN=-MD
+DEPGEN=-MMD
 -include ccan/*/*.d
 
 # Anything with C files needs building; dir leaves / on, sort uniquifies