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-4.2.0.orig/libstdc++-v3/include/c_std/std_cstdio.h	2006-12-07 10:33:51.000000000 +0100

+++ gcc-4.2.0/libstdc++-v3/include/c_std/std_cstdio.h	2007-05-20 01:05:38.000000000 +0200

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

 _GLIBCXX_END_NAMESPACE

-#if _GLIBCXX_USE_C99

+#if _GLIBCXX_USE_C99 || defined(__UCLIBC__)

 #undef snprintf

 #undef vfscanf