More explicit/robust way of finding Makefile.config.

lib/coq/Makefile

-# Incantation to find the root of the git repository.
-ROOT := $(shell while ! [ -d ./.git ] ; do cd .. ; done && pwd)
+# Incantation to find Makefile.config.
+ROOT := $(shell while ! [ -f ./Makefile.config ] ; do cd .. ; done && pwd)
 
 # Find TLC.
 include $(ROOT)/Makefile.config