build: pass down CC from make env

The default toolchain set up doesn't export CC/etc... into the children's
environment which means it'll be unset.  Pass it down from make's env by
default to fix that.

BUG=chromium:665649
TEST=`make` on ubuntu just works

Change-Id: Ic0bfcb5db6ade3dd598e31df7f43534eb8f3ae09
1 file changed