- Don't call enable_language before project.
See CMake docs for CMP0165.
- Remove boost-related cache variables.
It's too late to set them because Boost::headers has already been found.
- Don't build SDL3-test library.
Not used by shadps4.
- Remove /Zc:preprocessor from toml11's INTERFACE_COMPILE_OPTIONS.
Clang-cl does not support this flag and emits a lot of warnings.
- Remove dummy.cpp and make gcn a proper INTERFACE target.
