The whole class was a bad idea, why create something that's 99% similar
to another application and has just one platform-specific workaround? Of
course it resulted in this code being completely untested and not even
built anywhere, because it served a tiny insignificant use case.
To avoid losing all the code, I did my best in attempting to merge this
into the WindowlessEglApplication. But since, again, EGL isn't
really used on any Windows platform, I can't even say it builds
properly. Maybe not even the original code built.
Similar to the change done in Corrade, see the commit for details:
878624ac36
Wow, this is probably the most backwards-compatibility code I've ever
written. Can't wait until I can drop all that.
It limits the support for CMake 3.12+, but it's much less verbose and I
don't expect people to use ancient CMake versions with IDEs like Xcode
or VS anyway, so this should be fine.
I'm not really sure if the extra work and link dependencies are worth
the warning, but since I *need* to do something similar for Windows, why
not have it here as well.
Because these are static, there can be various problems when linking
them later, so better to test that as early as possible.
This was done in the magnum-bootstrap CI setup before, but that's often
too late to discover problems.