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

 reporter_name="Alexander BIGGA"

 reporter_url="http://sourceware.org/ml/crossgcc/2008-07/msg00000.html"

 reporter_comment=""