Added static linking of the windows runtime

[drowaudio]

No description provided.

[inkychris]

This method of customising the runtime library results in warnings for every source file compiled:

Command line warning D9025 : overriding '/MD' with '/MT'

The better way to set a default for this value (and also preserve the ability to customise it at configure time) is to use the following which was introduced in cmake 3.15 (so no need to bump the required cmake version for the project):

if(MSVC AND NOT CMAKE_MSVC_RUNTIME_LIBRARY)
    # Default to statically linking the runtime libraries
    set_property(TARGET pluginval PROPERTY MSVC_RUNTIME_LIBRARY "MultiThreaded$<$<CONFIG:Debug>:Debug>")
endif()

Techinically don't need the if(MSVC) in there since it will be ignored in non-MSVC environments, but it doesn't hurt and may seem more obvious to someone not familiar with this setting.

I suspect the build logs don't show any of these warnings because the build caching is swallowing them now and they showed up first time they ran in the pipeline.

[drowaudio]

Thanks for the tip, I've updated it here:
3253dad