Installing build dependencies directly on a host should be
considered an anti-pattern. Steal some of the bits from
rpm-ostree's PAPR bits.
Another benefit here is we avoid duplicating the build dependencies again, since
we start using `yum-builddep.
We only have one context now, `f25-primary`, rather than having Clang
show up as a separate thing, since IMO it's not worth its own context.
Colin Walters