This was done using macros before, which isn't the way we have things set up.
In theory that method should work, however if not all consumers include the
libroot_build headers properly, then it breaks in subtle but confusing ways,
which is not what we want at all.
Thanks to Jessica for advice.
Change-Id: Idd45df5547daecf8239932957088da03ddfccf87
648f0d5f08