[FEATURE] Collect extension selectors by minor version

[jgabaut]

At the moment, a series of extension selectors (flags for patches to 2.0) is loose in the code.

They should be collected by version, in the hope of some of them being squashed later.

A future version that drops patch-specific versioning for that given minor release would have a easier time handling them, if they're accessed by a common symbol.