gmp: Respect portage host cc variable

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

Gentoo PR: https://github.com/gentoo/gentoo/pull/16150

BUG=chromium:1088235
TEST=CQ

Change-Id: Id1364417e7854473f362547b550fe5601d152894
Reviewed-on: https://chromium-review.googlesource.com/c/chromiumos/overlays/portage-stable/+/2240121
Tested-by: Manoj Gupta <manojgupta@chromium.org>
Reviewed-by: George Burgess <gbiv@chromium.org>
2 files changed