cmake: Require CMake 3.15.0.
cmake: Require CMake 3.15.0.

This fixes CMP0091 policy, which wasn't been enacted, probably due to
being predicated by MSVC conditional, causing DLL MSVCRT being used
instead of statically linked one.

apitrace/apitrace,apitrace/apitrace,apitrace/apitrace,apitrace/apitrace