8001897: build-infra: misc adjustments to configure script

Reviewed-by: ohair
diff --git a/common/autoconf/Makefile.in b/common/autoconf/Makefile.in
index 629fadf..d1f1486 100644
--- a/common/autoconf/Makefile.in
+++ b/common/autoconf/Makefile.in
@@ -21,6 +21,7 @@
 # questions.
 #
 
-# Generated Makefile @DATE_WHEN_CONFIGURED@
+# This Makefile was generated by configure @DATE_WHEN_CONFIGURED@
+# GENERATED FILE, DO NOT EDIT
 SPEC:=@OUTPUT_ROOT@/spec.gmk
 include @SRC_ROOT@/common/makefiles/Makefile