cmake: Modernize compile settings; require C++14

Clean up QuasselCompileSettings.cmake, treat GNU and Clang the same.
Use add_compile_options() instead of CMAKE_CXX_FLAGS. Curate the list
of compiler flags, and add sensible linker flags, too.

Check explicitly for required compiler features using CMake's
target_compile_features(), rather than hard-coding compiler versions.
The set of required features basically implies that we now require
a C++14 compiler (even though the new stuff isn't used in the code
yet).