These changes modernize the CMake build system to (hopefully) match newer best practices
* Library linking is simpler and more automatic because of using imported targets
* Multi-configuration builds should be supported (Ninja Multi-Config, Visual Studio, etc. generators)
* Clean up build options using cmake_dependent_option
* Let CMake do its job in more cases, like finding the math/dl libraries and detecting and enabling LTO support
* Remove platform-specific kludges like the Fedora/flatpak LTO workaround and a bunch of Windows stuff
* Simplify Windows static builds
* Consistent formatting
* Use libepoxy to load in OpenGL functions
Prevents having to load them in manually
* Install libepoxy in the CI
* Do not link OpenGL libraries, libepoxy opens them itself
* Add libepoxy to build instructions
Nadia Holmquist Pedersen