* 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.
This moves all build options to their own file to clean up the main
CMakeLists.txt. In addition, this upgrades the minimum required CMake
version and changes the build to use CMAKE_MSVC_RUNTIME_LIBRARY rather
than hardcoding the MSVC ABI flag.
Fabrice de Gans