protobuf: Respect portage host cc variable

Pass CC_FOR_BUILD and CXX_FOR_BUILD to econf.
Otherwise it invokes gcc/g++ instead of portage specified HOST/BUILD CC.

Gentoo PR: https://github.com/gentoo/gentoo/pull/16181

BUG=chromium:979916, chromium:991812, chromium:1093633
TEST=CQ

Change-Id: I538d2be016a3935cf66e74c2511ecdb880759f6b
Reviewed-on: https://chromium-review.googlesource.com/c/chromiumos/overlays/portage-stable/+/2238521
Tested-by: Manoj Gupta <manojgupta@chromium.org>
Reviewed-by: Mike Frysinger <vapier@chromium.org>
Commit-Queue: Manoj Gupta <manojgupta@chromium.org>
1 file changed