Fix MSVC warning C4996: 'getenv': This function or variable may be unsafe

[ghosty141]

warning C4996: 'getenv': This function or variable may be unsafe. Consider using _dupenv_s instead. To disable deprecation, use _CRT_SECURE_NO_WARNINGS. See online help for details.

It would be great if this could either get fixed or ignored by adding _CRT_SECURE_NO_WARNINGS to the compile_definitions. This would make backward-cpp warning free!