Fixes to CMake for MSVC build

[m4x1m1l14n]

MSVC build was failing under Visual Studio 2017 with CMake v3.15.5 and v3.16.2 generated project files.

Compiler cl.exe was failing with following error code for all GCC specific compiler flags

cl : Command line error D8021: invalid numeric argument '/Wextra'

CHECK_CXX_COMPILER_FLAG() evaluates with GCC specific compiler flags to positive result.

[artwyman]

Thanks for your effort! This repo is no longer supported by Dropbox and about to be archived. Since CMAKE was community-submitted and not something I was set up to test, I can't validate if this is the right approach for all users. So I'm not going to try to merge it at the last minute, but I'll leave it open for reference by anyone who encounters the same problem.