Use #if !HAVE_DECL...' instead of #ifndef HAVE_DECL..'

now that autoconf always defines the HAVE_DECL_ symbols.

Arrange for cpp to fail if the configure-time
declaration check was not run.

lib/human.c

@@ -1,5 +1,5 @@
 /* human.c -- print human readable file size
-   Copyright (C) 1996, 1997, 1998, 1999 Free Software Foundation, Inc.
+   Copyright (C) 1996, 1997, 1998, 1999, 2000 Free Software Foundation, Inc.
 
    This program is free software; you can redistribute it and/or modify
    it under the terms of the GNU General Public License as published by
@@ -44,6 +44,9 @@
 #endif
 
 #ifndef HAVE_DECL_GETENV
+'this configure-time declaration test was not run'
+#endif
+#if !HAVE_DECL_GETENV
 char *getenv ();
 #endif