Remove GREP_OPTIONS is deprecated warning

$ grep --version
grep (GNU grep) 2.21
Copyright (C) 2014 Free Software Foundation, Inc.
License GPLv3+: GNU GPL version 3 or later <http://gnu.org/licenses/gpl.html>.
This is free software: you are free to change and redistribute it.
There is NO WARRANTY, to the extent permitted by law.

Written by Mike Haertel and others, see <http://git.sv.gnu.org/cgit/grep.git/tree/AUTHORS>.

The new version of grep(version 2.21) seems deprecated the usage of GREP_OPTIONS. When I execute grep command in emacs, the warning: 'grep: warning: GREP_OPTIONS is deprecated; please use an alias or script' will always show as below:-

-*- mode: grep; default-directory: "c:/emacs/" -*-
Grep started at Mon Feb 16 09:53:48

find . -type f -print0 | xargs -0 -e grep -n -e GREP_OPTIONS
grep: warning: GREP_OPTIONS is deprecated; please use an alias or script
grep: warning: GREP_OPTIONS is deprecated; please use an alias or script
grep: warning: GREP_OPTIONS is deprecated; please use an alias or script
grep: warning: GREP_OPTIONS is deprecated; please use an alias or script
grep: warning: GREP_OPTIONS is deprecated; please use an alias or script
grep: warning: GREP_OPTIONS is deprecated; please use an alias or script
./share/emacs/24.4/lisp/progmodes/grep.el:80:markers for highlighting and GREP_OPTIONS to add the --color
./share/emacs/24.4/lisp/progmodes/grep.el:470:    (setenv "GREP_OPTIONS"
./share/emacs/24.4/lisp/progmodes/grep.el:471:	    (concat (getenv "GREP_OPTIONS")
Binary file ./share/emacs/24.4/lisp/progmodes/grep.elc matches
grep: warning: GREP_OPTIONS is deprecated; please use an alias or script

Grep exited abnormally with code 123 at Mon Feb 16 09:53:50

When I dug into the elisp code c:/emacs/share/emacs/24.4/lisp/progmodes/grep.el::470, I find GREP_OPTIONS are used:

(setenv "GREP_OPTIONS"
    (concat (getenv "GREP_OPTIONS")
        " --color=" (if (eq grep-highlight-matches 'always)
                "always" "auto")))

You can just comment above lines of code, the world will be quiet again. Remember to byte compile grep.el again.

-*- mode: grep; default-directory: "c:/emacs/" -*-
Grep started at Mon Feb 16 09:53:48

find . -type f -print0 | xargs -0 -e grep -n -e GREP_OPTIONS
./share/emacs/24.4/lisp/progmodes/grep.el:80:markers for highlighting and GREP_OPTIONS to add the --color
./share/emacs/24.4/lisp/progmodes/grep.el:470:;;    (setenv "GREP_OPTIONS"
./share/emacs/24.4/lisp/progmodes/grep.el:471:;;	    (concat (getenv "GREP_OPTIONS")
Binary file ./share/emacs/24.4/lisp/progmodes/grep.elc matches