Add a config.h to ease the transition. Now if you include config.h
you get a warning instead of a failure:
In file included from System/Posix/Internals.hs:24:
/playpen/ghc/nightly/HEAD-cam-02-unx/i386-unknown-linux/ghc/includes/config.h:4: warning: #warning config.h is deprecated; please use ghcconfig.h instead
--- /dev/null
+#ifndef __CONFIG_H__
+#define __CONFIG_H__
+
+#warning config.h is deprecated; please use ghcconfig.h instead
+#include "ghcconfig.h"
+
+#endif