Quote bash comparison arguments

/cc @chyndman

Author: José Valim

Makefile

@@ -175,7 +175,7 @@ test_doc_test: compile
 test_stdlib: compile
 	@ echo "==> elixir (exunit)"
 	$(Q) exec epmd & exit
-	$(Q) if [ $(OS) = Windows_NT ]; then \
+	$(Q) if [ "$(OS)" = "Windows_NT" ]; then \
 		cd lib/elixir && cmd //C call ../../bin/elixir.bat -r "test/elixir/test_helper.exs" -pr "test/elixir/**/*_test.exs"; \
 	else \
 		cd lib/elixir && ../../bin/elixir -r "test/elixir/test_helper.exs" -pr "test/elixir/**/*_test.exs"; \