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/os_unix.h b/src/os_unix.h
index ca2ce70..8919ff0 100644
--- a/src/os_unix.h
+++ b/src/os_unix.h
@@ -369,8 +369,13 @@
 # define CLEAN_RUNTIMEPATH      "$VIM/vimfiles,$VIMRUNTIME,$VIM/vimfiles/after"
 #else
 # ifdef RUNTIME_GLOBAL
-#  define DFLT_RUNTIMEPATH	"~/.vim," RUNTIME_GLOBAL ",$VIMRUNTIME," RUNTIME_GLOBAL "/after,~/.vim/after"
-#  define CLEAN_RUNTIMEPATH	RUNTIME_GLOBAL ",$VIMRUNTIME," RUNTIME_GLOBAL "/after"
+#  ifdef RUNTIME_GLOBAL_AFTER
+#   define DFLT_RUNTIMEPATH	"~/.vim," RUNTIME_GLOBAL ",$VIMRUNTIME," RUNTIME_GLOBAL_AFTER ",~/.vim/after"
+#   define CLEAN_RUNTIMEPATH	RUNTIME_GLOBAL ",$VIMRUNTIME," RUNTIME_GLOBAL_AFTER
+#  else
+#   define DFLT_RUNTIMEPATH	"~/.vim," RUNTIME_GLOBAL ",$VIMRUNTIME," RUNTIME_GLOBAL "/after,~/.vim/after"
+#   define CLEAN_RUNTIMEPATH	RUNTIME_GLOBAL ",$VIMRUNTIME," RUNTIME_GLOBAL "/after"
+#  endif
 # else
 #  define DFLT_RUNTIMEPATH	"~/.vim,$VIM/vimfiles,$VIMRUNTIME,$VIM/vimfiles/after,~/.vim/after"
 #  define CLEAN_RUNTIMEPATH	"$VIM/vimfiles,$VIMRUNTIME,$VIM/vimfiles/after"