Made warnings disappear about possibly missing external folder when building.

Makefile

@@ -8,7 +8,7 @@ COQ_MAKEFILE = $(COQBIN)coq_makefile
 endif
 
 # We don't want to manually specify all buildable vfiles, and we also have external dependencies..
-HAS_EXTERNAL := $(shell if [ -n "$$(find ./external/ -type f)" ] ; then echo "1" ; else echo "0" ; fi)
+HAS_EXTERNAL := $(shell if [ -d "./external" ] && [ -n "$$(find ./external/ -type f)" ] ; then echo "1" ; else echo "0" ; fi)
 # above if statement checks if there are regular files in that directory
 # this should indicate having run `git submodule init --update --recursive`
 ifeq ("$(HAS_EXTERNAL)", "1")