cmake: Prefer CMAKE_MSVC_RUNTIME_LIBRARY for runtime library selection
This is available since cmake 3.15, see https://cmake.org/cmake/help/latest/variable/CMAKE_MSVC_RUNTIME_LIBRARY.html.
We can actually figure out the correct value of CMAKE_MSVC_RUNTIME_LIBRARY ourselves, as we don't support all combinations, see 20bb9f47 (appveyor.yml: Fix MSVC runtime library selection, 2022-07-08).