With older builds CMake will error out with `CXX_STANDARD is set to invalid value '17'`
CXX_STANDARD 17 was added on CMake 3.8.X as found in the 3.8.2 documentation: https://cmake.org/cmake/help/v3.8/prop_tgt/CXX_STANDARD.html
Also removes leftover from old VS2015/2017 mixed instructions, removes "Not GCC 6.1" since we require 7.3+ now and bumps Xcode to 10. Proper MacOS instructions will be added afterwards by someone else.
Moves the compiler check out of rpcs3/CMakeLists.txt and adds them
into rpcs3/cmake_modules/ConfigureCompiler.cmake
Add better compiler checks and eliminate a TODO
