RFC: Feature macros (aka USE flags)

[Petr Šabata]

Based on the recent discussions around %fedora/%rhel macros and ELN,
and %bcond generally being confusing to work with, I came up with a
distribution-wide feature that defines generic feature keywords and
associated helper macros that packages can check in build-time
conditionals.

The key advantage here is the defaults are defined by the buildroot,
not the package. The package is just a building block.

I'd like some input to improve this and unless this turns out to be a
really bad idea, I intend to submit it as a change proposal. Even
though the more packages use it the more beneficial it gets, it's, of
course, perfectly optional.

Details in the gist:
https://gist.github.com/contyk/0aaaaf0585c57976ca18a293b3566408

Cheers,
P