zandronum: exlib fixes

- export_exlib_phases
- use ever at_least instead of ever is_scm
- consistency
- warning about virtual/libressl
slade: bump to 3.1.0 and exlib fixes

- export_exlib_phases
- move rev exparam generation in exlib
- bump patch (still far from complete)
- remove ftgl dep, allowed by patch
- revise SFML dep