2.7 features and CI

[Joshua Root]

It's been well over the usual 2 weeks we normally wait before using 
features introduced in a new release. But CI is stuck on 2.6.4 and will 
fail for ports that use the new features. Of course we should really get 
CI updated to the new version, but nobody has managed to do that yet, so 
in the meantime we have this dilemma.

New variables in 2.7 include:
configure.sysroot
macos_version & macos_version_major
universal_possible

- Josh