Support approximate matching of MSVC toolset

Consumers can select a specific version of the toolset to use by setting the `VS_PLATFORM_TOOLSET_VERSION` property, but it has to be an exact match to an installed version (e.g., `14.36.32532`). `vcvarsall.bat` is precedence for specifying a two-component version (e.g. `14.36`). `VS_PLATFORM_TOOLSET_VERSION` should accommodate similar flexibility.