This is VS2017 Preview. It makes sure the version is detected properly and the INSTALL target then correctly installs to x64/vc15 (the same as MSVC1910).
Without the fix, OpenCV will compile and silently copy the compiled libs/executables to a location not prefixed with arch/vc14, and OpenCV won't be picked up by find_package and OpenCVConfig.cmake.
Make detection of x64 using the gcc's target triplet identical
to the one used in cmake/OpenCVDetectCXXCompiler.cmake.
Otherwise, MinGW-w64 setups will always be treated as x64 since
they contain "w64" as vendor key.