honor COQBIN

Makefile

+ifeq "$(COQBIN)" ""
+  COQBIN=$(dir $(shell which coqtop))/
+endif
+
 # Forward most targets to Coq makefile (with some trick to make this phony)
 %: Makefile.coq phony
 	+@make -f Makefile.coq $@
@@ -15,7 +19,7 @@ clean: Makefile.coq
 # Create Coq Makefile. POSIX awk can't do in-place editing, but coq_makefile wants the real
 # filename, so we do some file gymnastics.
 Makefile.coq: _CoqProject Makefile awk.Makefile
-	coq_makefile -f _CoqProject -o Makefile.coq
+	$(COQBIN)/coq_makefile -f _CoqProject -o Makefile.coq
 	mv Makefile.coq Makefile.coq.tmp && awk -f awk.Makefile Makefile.coq.tmp > Makefile.coq && rm Makefile.coq.tmp
 
 # Install build-dependencies