Add additional C++ CI configurations

[metopa]

This issue documents the progress on adding CI configurations for supported C++ compilers.

• [x] Default GCC on Ubuntu
• [ ] GCC 7 on Ubuntu
• [ ] Clang 5 on Ubuntu
• [x] Default Clang on macOS
• [x] Default MSVC on Windows
• [ ] MSVC 14.1+ (VS 2017) on Windows
• [ ] MinGW (MSYS2) on Windows
• [ ] Cygwin on Windows
• [ ] LXSS GCC on Windows