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

 #!/bin/sh

 canonicalizedname=$(readlink -nm "${0}")

 dirname="$(dirname "${canonicalizedname}")"

 basename="$(basename "${canonicalizedname}")"

 ld_lib_path="$(dirname "${dirname}")/lib"

 case ":${LD_LIBRARY_PATH}:" in

     *":${ld_lib_path}:"*)   ;;

     *)  LD_LIBRARY_PATH="${ld_lib_path}${LD_LIBRARY_PATH:+:${LD_LIBRARY_PATH}}";;

 esac

 export LD_LIBRARY_PATH

 exec "${dirname}/.${basename}" "$@"