Output to confdefs.h directly.

m4/gnu-source.m4

@@ -1,4 +1,4 @@
-#serial 1
+#serial 2
 # Make sure _GNU_SOURCE is defined where necessary: as early as possible
 # for configure-time tests, as well as for every source file that includes
 # config.h.
@@ -11,11 +11,11 @@ AC_DEFUN(AC__GNU_SOURCE,
   # configure-time compile tests.
   # This definition must be emitted (into confdefs.h) before any
   # test that involves compilation.
-  AC_DEFINE(_GNU_SOURCE_KLUDGE, [1
+  cat >>confdefs.h <<\EOF
 #ifndef _GNU_SOURCE
 # define _GNU_SOURCE
-#endif], [This definition has a four-line value, the last three of which
-ensure that _GNU_SOURCE is defined in confdefs.h.])
+#endif
+EOF
 
   # Emit this code into config.h.in.
   # The ifndef is to avoid redefinition warnings.