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 strace-4.5.19.orig/system.c strace-4.5.19/system.c

--- strace-4.5.19.orig/system.c	2009-06-01 11:59:11.000000000 +0200

+++ strace-4.5.19/system.c	2009-10-28 18:47:18.000000000 +0100

@@ -1654,7 +1654,9 @@

 	{ CTL_KERN, "CTL_KERN" },

 	{ CTL_VM, "CTL_VM" },

 	{ CTL_NET, "CTL_NET" },

+#ifdef CTL_PROC

 	{ CTL_PROC, "CTL_PROC" },

+#endif

 	{ CTL_FS, "CTL_FS" },

 	{ CTL_DEBUG, "CTL_DEBUG" },

 	{ CTL_DEV, "CTL_DEV" },