Complete overhaul in Continuous Integration

[geneotech]

There are several problems with the current state of CI:

• [x] The build doesn't pass under gcc-7.2 because of a compiler bug - we'll therefore need to build it under gcc-7.3.
  • It now builds under gcc 7.3.1.
• [ ] It would be great if clang + llvm + libcxx + libcxxabi could be downloaded instead of rebuilt each time, so that we are environmentally friendly.
• [ ] Windows build doesn't pass, but whatevs!
  • This is really more of a code issue, we'll probably just wait until they introduce the C++17 features for MSVC.
  • Or we can try building Hypersomnia for Windows with help of clang.

[kacperduras]

I propose this combo: matrix builds (with macOS) + build stages.

[geneotech]

Could you elaborate maybe? We already have a build matrix on Travis, wherein one job is specified for clang and a separate one for gcc. What's that about Mac OS?

[kacperduras]

We already have a build matrix on Travis, wherein one job is specified for 'clang' and a separate one for 'gcc.'

Ok, but only for Linux. Builds on Windows are available on AppVeyor, but where are the builds on macOS?

[geneotech]

For the time being, it is enough that, under both platforms, correct builds are produced with LLVM 7.