The Monocypher vendored C files use -Wno-unused-function / -Wno-unused-parameter
compile flags that are GCC/Clang-only syntax. MSVC chokes on them with:
cl : Command line error D8021 : invalid numeric argument '/Wno-unused-function'
Wrap the set_source_files_properties call in if (NOT MSVC) so the flags
are applied on macOS/Linux but skipped on Windows. The swagger CMakeLists.txt
uses the same pattern.