config: fix generated config files when GREP_OPTIONS set --color=always

grep is used when generating some .in files (in config.gen). If GREP_OPTIONS
contains --color=always, then the generated files will contains some escape
sequences setting the color, and mconf would choke on those escape sequences.

Always set and export an empty GREP_OPTIONS to avoid that.

Issue reported by Kevin Kirkup <kevin dot kirkup at gmail dot com> :
http://sourceware.org/ml/crossgcc/2009-12/msg00026.html

diff -dur binutils-2.17.old/bfd/opncls.c binutils-2.17/bfd/opncls.c

--- binutils-2.17.old/bfd/opncls.c	2006-03-16 13:20:16.000000000 +0100

+++ binutils-2.17/bfd/opncls.c	2007-05-01 18:26:11.000000000 +0200

@@ -158,6 +158,13 @@

 {

   bfd *nbfd;

   const bfd_target *target_vec;

+  struct stat s;

+

+  if (stat (filename, &s) == 0)

+    if (S_ISDIR(s.st_mode)) {

+      bfd_set_error (bfd_error_file_not_recognized);

+      return NULL;

+    }

   nbfd = _bfd_new_bfd ();

   if (nbfd == NULL)