CMake: Enable /MP for MSVC toolchain

Robert Daileyrequested to merge
rcdailey/cmake:msvc-parallel-build-17696 into master

A new cache option named CMake_MSVC_PARALLEL is now available that can be used to control the usage of /MP to the MSVC compiler. This enables parallelized builds on a per-translation unit basis. To enable /MP, specify value ON to the option. Using an integral non-zero value will control the specific number of threads used with the /MP option, as opposed to letting the toolchain decide for you.

Fixes #17696 (closed)

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