CI: C++14/17

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.