Add CMake support to CMock

[orjangj]

No description provided.

[orjangj]

This PR only resolves item nr. 1 from #505 (adding basic CMake support to CMock). Maybe #505 should be split up into separate tickets.

The CMake integration in CMock is almost a direct copy-paste from https://github.com/ThrowTheSwitch/Unity. Not sure if I should give credit in the CMakeLists.txt file or not.

CMake doesn't have the same fallback capabilities as Meson, but I've tried to mimic the behavior with:

if(NOT TARGET unity::framework)
    add_subdirectory(vendor/unity)
endif()