patch 8.1.0612: cannot use two global runtime dirs with configure

Problem:    Cannot use two global runtime dirs with configure.
Solution:   Support a comma in --with-global-runtime. (James McCoy,
            closes #3704)
diff --git a/src/config.h.in b/src/config.h.in
index 8352563..4a5cb09 100644
--- a/src/config.h.in
+++ b/src/config.h.in
@@ -453,9 +453,12 @@
 /* Define if you want to include terminal emulator support. */
 #undef FEAT_TERMINAL
 
-/* Define default global runtime path */
+// Define default global runtime path.
 #undef RUNTIME_GLOBAL
 
+// Define default global runtime after path.
+#undef RUNTIME_GLOBAL_AFTER
+
 /* Define name of who modified a released Vim */
 #undef MODIFIED_BY