Makefile: re-generate per-suite Makefile(s) if needed

Make per-suite Makefile depend on scripts generating it, so that
make will re-generate Makefile if any of its scripts has changed.
Otherwise, Makefile(s) are not re-generated automatically.

Change-Id: I8b73bf8e4c2f6e008eab9581f8c6840b22a5bd6a
diff --git a/Makefile b/Makefile
index 5a35e6e..cbb8083 100644
--- a/Makefile
+++ b/Makefile
@@ -87,7 +87,7 @@
 all: $(foreach dir,$(SUBDIRS),$(dir)/all)
 
 define DIR_Makefile_template
-$(1)/Makefile:
+$(1)/Makefile: $(1)/gen_links.sh $(1)/regen_makefile.sh
 	(cd $(1) && ./gen_links.sh && ./regen_makefile.sh)
 endef