Accommodate more variables being set by callers (for MT, RC, C- and C++- compilers)

[MarkSchofield]

#148 highlights opportunities for using WindowsToolchains for finding, say, Windows SDKs, but using other tooling - like LLVM's 'mt' or 'rc' - for compilation steps. That's a bit tricky at the minute, because Windows.Kits.cmake/Windows.Clang.toolchain.cmake/Windows.MSVC.toolchain.cmake make too many assumptions. This change;

1. Updates Windows.Kits.cmake to only set CMAKE_MT or CMAKE_RC_COMPILER[_INIT] if values aren't already set. This allows callers to prefer llvm-mt or llvm-rc.
2. Updates Windows.Clang.toolchain.cmake to not set CMAKE_C_COMPILER or CMAKE_CXX_COMPILER if they have already been set.
3. Adds the appropriate variables to CMAKE_TRY_COMPILE_PLATFORM_VARIABLES to ensure that the caller-provided values are propagated into 'try compile'.