libidn2: Respect portage host cc

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

Gentoo PR: https://github.com/gentoo/gentoo/pull/16151

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

Change-Id: Ib38b77be2a4ad27314409d2a07943383a83ece24
Reviewed-on: https://chromium-review.googlesource.com/c/chromiumos/overlays/portage-stable/+/2240125
Tested-by: Manoj Gupta <manojgupta@chromium.org>
Reviewed-by: George Burgess <gbiv@chromium.org>
1 file changed