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)