mkhelp: generate code for --disable-manual as well

This allows configure --disable-manual to run and build without having
to regenerate the src/tool_hugehelp.c file which otherwise is necessary
since we ship tarballs with that file present.

Reported-by: Remi Gacogne
Bug: http://curl.haxx.se/bug/view.cgi?id=1350

src/mkhelp.pl

@@ -6,7 +6,7 @@
 #                            | (__| |_| |  _ <| |___
 #                             \___|\___/|_| \_\_____|
 #
-# Copyright (C) 1998 - 2011, Daniel Stenberg, <daniel@haxx.se>, et al.
+# Copyright (C) 1998 - 2014, Daniel Stenberg, <daniel@haxx.se>, et al.
 #
 # This software is licensed as described in the file COPYING, which
 # you should have received as part of this distribution. The terms
@@ -256,6 +256,10 @@ foot();
 
 sub foot {
   print <<FOOT
+#else /* !USE_MANUAL */
+/* built-in manual is disabled, blank function */
+#include "tool_hugehelp.h"
+void hugehelp(void) {}
 #endif /* USE_MANUAL */
 FOOT
   ;