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
(transplanted from a5020a1facffc046d23d39a71a1b8089ca84beac)

 --- gcc-4.1.2.orig/libstdc++-v3/configure	2007-03-04 23:32:31.000000000 +0100

 +++ gcc-4.1.2/libstdc++-v3/configure	2007-03-04 23:33:54.000000000 +0100

 @@ -7324,6 +7324,9 @@

  cat >>conftest.$ac_ext <<_ACEOF

  /* end confdefs.h.  */

  #include <complex.h>

 +#ifdef __UCLIBC__

 +#error ugly hack to make sure configure test fails here for cross until uClibc supports the complex funcs

 +#endif

  int

  main ()

  {