[Linux/BSD] Add dynamically loaded library version checks.

[bruvzg]

Adds min. version checks (or function availability checks in case there's no API to check version) for the dynamically loaded libraries.

Splits XKBcommon function checks in two (for version 0.5 and 0.8) to support dead key composition on older distros (0.8 is required to decode Unicode key labels only).

Most of the min. versions correspond to Ubuntu 16.04 (the oldest distro on which I was able to build and test Godot 4).

[Riteo]

I see that you're checking for every library's major version as more-than-or-equal a target value. Major version bumps usually mean breaking changes. Are you sure that's a good idea?

[bruvzg]

Major version bumps usually mean breaking changes. Are you sure that's a good idea?

If it's a backward incompatible change, soname should also change, so it won't load the library at all.

[Riteo]

@bruvzg

If it's a backward incompatible change, soname should also change, so it won't load the library at all.

Right, all right then.

[akien-mga]

Thanks!