Allow overriding MSVC runtime library setting in CMake

[emuell]

I'm working on Rust bindings for FluCoMa, based on flucoma-core. See https://github.com/emuell/flucoma-rs

This still is a WIP, but works great so far, with one little caveat: Rust by default uses the Multi-threaded DLL runtime with msvc targets. This can be overridden, but is quite awkward and requires all users of the flucoma-rs crate to override the runtime as well which may cause various other problems.

flucoma-core right now does not allow overriding the CMAKE CMAKE_MSVC_RUNTIME_LIBRARY variables. This PR changes this. The default still is /MT, so this should not break anything else.

[tremblap]

@weefuzzy is currently working on the cmake stuff, so maybe this is timely to be added?

[emuell]

Any chance of getting that included? I can work around this by using a fork, but it would be great to let the Rust wrapper use the official sources.

[weefuzzy]

This looks ok.

Note that it will be a temporary tweak, because I don't think we should be setting this variable in core in the first place (and if we do, probably in a preset rather than CMakeLists). But my CMake changes won't appear for a while.

@tremblap, I don't think this will have any side effects for us, but worth checking the next nightlies for Max (in particular) on Windows, just in case.