patch 9.0.0594: Makefile error message causes a shell error

Problem:    Makefile error message causes a shell error.
Solution:   Put the message in single quotes. (closes #11232)
diff --git a/src/Makefile b/src/Makefile
index 668a87f..14601a2 100644
--- a/src/Makefile
+++ b/src/Makefile
@@ -3578,7 +3578,7 @@
 
 
 Makefile:
-	@echo The name of the makefile MUST be "Makefile" (with capital M)!!!!
+	@echo 'The name of the makefile MUST be "Makefile" (with capital M)!'
 
 
 ###############################################################################