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)

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.