env.bash: Check for presence of make/gmake

Without this, a missing gmake will result in bogus error "Did not find
Go environment variables".

R=golang-dev, rsc
CC=golang-dev
https://golang.org/cl/5369066

src/env.bash

@@ -59,6 +59,7 @@ PROGS="
 	gcc
 	grep
 	ls
+	$MAKE
 	mkdir
 	mv
 	pwd