Emacs won't assume grep supports GREP_OPTIONS

Emacs won't assume grep supports GREP_OPTIONS

This upstream patch has been added:

  Don't assume 'grep' supports GREP_OPTIONS.

  The GREP_OPTIONS environment variable is planned to be marked
  obsolescent in GNU grep, due to problems in its use, so stop
  relying on it.
  * progmodes/grep.el (grep-highlight-matches): Document this.
  (grep-process-setup): Do not set GREP_OPTIONS.
  (grep-compute-defaults): Use an explicit --color option if supported.

Origin: upstream, commit: 2e4c2fe2787785a421f256541de642976e9bd90b
Bug: http://debbugs.gnu.org/19247
Bug-Debian: http://bugs.debian.org/793741
Added-by: Rob Browning <rlb@defaultvalue.org>