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

     1.1 --- a/ct-ng.in	Sat Dec 19 13:00:51 2009 +0100
     1.2 +++ b/ct-ng.in	Tue Dec 29 21:16:51 2009 +0100
     1.3 @@ -32,6 +32,10 @@
     1.4  # bash.
     1.5  export SHELL=$(bash)
     1.6  
     1.7 +# GREP_OPTIONS=--color=always will break the generated .in files
     1.8 +# We do not need any GREP_OPTIONS anyway, so set it to empty.
     1.9 +export GREP_OPTIONS=
    1.10 +
    1.11  # Make the restart/stop steps availabe to scripts/crostool-NG.sh
    1.12  export CT_STOP:=$(STOP)
    1.13  export CT_RESTART:=$(RESTART)