Fixing typo in Makefile.

Closes #742.
diff --git a/Makefile b/Makefile
index 278824a..ddae1ce 100644
--- a/Makefile
+++ b/Makefile
@@ -2081,7 +2081,7 @@
 	$(Q) $(INSTALL) etc/roots.pem $(prefix)/share/grpc/roots.pem
 
 verify-install:
-ifeq ($(SYSTEM_OK),true)
+ifeq ($(INSTALL_OK),true)
 	@echo "Your system looks ready to go."
 	@echo
 else