Correct build when using grep with coloured output

Summary:
With Linux (Archlinux) it fails to detect llvm version when grep colors are activated via environment variable.
GREP_OPTIONS=--color=always

Alternative to this patch, would be to modify build/envsetup.sh to properly detect grep color settings in user environment and set it to auto.

Test Plan: verified

Reviewers: randall.rushing, synergy

Reviewed By: synergy

Differential Revision: http://review.codefirex.com/D99