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

 diff -durN gcc-4.4.0.orig/libgomp/configure gcc-4.4.0/libgomp/configure

 --- gcc-4.4.0.orig/libgomp/configure	2009-04-21 11:08:08.000000000 +0200

 +++ gcc-4.4.0/libgomp/configure	2009-05-27 21:38:19.000000000 +0200

 @@ -3334,7 +3334,7 @@

  # Add -Wall -Werror if we are using GCC.

  if test "x$GCC" = "xyes"; then

 -  XCFLAGS="$XCFLAGS -Wall -Werror"

 +  XCFLAGS="$XCFLAGS -Wall"

  fi

  # Find other programs we need.