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 -durN gcc-4.2.1.orig/libstdc++-v3/include/c_std/std_cstdio.h gcc-4.2.1/libstdc++-v3/include/c_std/std_cstdio.h

 --- gcc-4.2.1.orig/libstdc++-v3/include/c_std/std_cstdio.h	2006-12-07 10:33:51.000000000 +0100

 +++ gcc-4.2.1/libstdc++-v3/include/c_std/std_cstdio.h	2007-08-03 20:33:24.000000000 +0200

 @@ -144,7 +144,7 @@

  _GLIBCXX_END_NAMESPACE

 -#if _GLIBCXX_USE_C99

 +#if _GLIBCXX_USE_C99 || defined(__UCLIBC__)

  #undef snprintf

  #undef vfscanf