CI: C++14/17

[ax3l]

Add some compile matrix entries that already build with C++14 and C++17 to see if we hit incompatibilities.

Important since e.g. conda-forge already builds with C++14 and C++17 chains.

We might also need to remove CXX_EXTENSIONS=ON forcing in our CMakeLists.txt because that should actually be controlled externally. If we remove it, we should document that we, CMake-typically, don't just build with -std=c++11 but with the newest found standard such as -std=c++1z or -std=gnu++17, in case the compiler supports it.