Update Makefile

Makefile

-# Forward most targets to Coq makefile (with some trick to make this phony)
-%: Makefile.coq phony
-	+@make -f Makefile.coq $@
-
 all: Makefile.coq
 	+@make -f Makefile.coq all
 .PHONY: all
 
+# Permit local customization
+-include Makefile.local
+
+# Forward most targets to Coq makefile (with some trick to make this phony)
+%: Makefile.coq phony
+	+@make -f Makefile.coq $@
+
 clean: Makefile.coq
 	+@make -f Makefile.coq clean
 	find theories tests \( -name "*.d" -o -name "*.vo" -o -name "*.aux" -o -name "*.cache" -o -name "*.glob" -o -name "*.vio" \) -print -delete || true
@@ -43,10 +46,10 @@ build-dep: builddep
 .PHONY: build-dep
 
 
-# Some files that do *not* need to be forwarded to Makefile.coq
-Makefile: ;
-_CoqProject: ;
-opam: ;
+# Some files that do *not* need to be forwarded to Makefile.coq.
+# ("::" lets Makefile.local overwrite this.)
+Makefile Makefile.local _CoqProject $(OPAMFILES):: ;
+
 
 # Phony wildcard targets
 phony: ;