Correct build when using grep with coloured output

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.

Change-Id: Icebf205d80f4dea927aeb7f9fc800d29050d3498