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

http://sourceware.org/ml/binutils/2007-07/msg00401.html

http://sourceware.org/bugzilla/show_bug.cgi?id=4970

--- binutils/configure

+++ binutils/configure

@@ -5601,6 +5601,7 @@ case "${host}" in

   *-*-mingw* | *-*-cygwin ) RPATH_ENVVAR=PATH ;;

   *) RPATH_ENVVAR=LD_LIBRARY_PATH ;;

 esac

+RPATH_ENVVAR="cant_touch_this_nah_nah_nah"

 # On systems where the dynamic library environment variable is PATH,

 if test "$RPATH_ENVVAR" = PATH; then