points at the "latest" branch and one which points at the "quarterly"
branch. Install the "latest" version unless overridden via the newly
added PKGCONFBRANCH variable.
This does not change user-visible behaviour (assuming said vairable is
not set) but will make it easier to change the defaults in the future --
on stable branches we will want "latest" on x86 but "quarterly" elsewhere.
Discussed with: gjb
MFC after: 3 days
X-MFC: After MFCing this I'll make a direct commit to stable/* to
switch non-x86 architectures to "quarterly".
b583215077