Off by default, build behaves normally.
WITH_META_MODE we get auto objdir creation, the ability to
start build from anywhere in the tree.
Still need to add real targets under targets/ to build packages.
Differential Revision: D2796
Reviewed by: brooks imp
to ease any rework of how clang is built to take arm64 in to account.
Submitted by: andrew
Reviewed by: andrew, emaste
Differential Revision: https://reviews.freebsd.org/D1446