With CMake 3.15, a new mechanism was added to control which CRT
libraries link to. Because the minimum CMake version is above 3.15,
googletest can leverage it in the implementation of
gtest_force_shared_crt.
If CMAKE_MSVC_RUNTIME_LIBRARY is already defined, googletest will
respect its value rather than replace it, allowing for users to
customize the exact CRT used by googletest.
