Apple compiler ID is defined as `AppleClang`. Rather than explicitly
adding every single compiler ID, this changes the build to try to apply
settings for GCC for every non-MSVC-like compilers.
* Use add_compile_definitions everywhere.
* Remove unused intermediate target.
* Add ASAN support for MSVC.
* Remove duplicate option definitions.
* Configure MinGW separately from the toolchain
* Move toolchain-specific options to their own files.
* Clean up and modernize the use of toolchain options.
* Use modern cmake LTO support.
* Remove dead cmake code and cmake functions available in upstream cmake.
* Update README.md to remove references to removed build options.
Fabrice de Gans