Simplify cxx_default by reusing functionality from gxx_common and clang_common

@mortenvp @jpihl I made some improvements to cxx_default: it uses the functions defined in gxx_common and clang_common to find the compilers, and it will always give a proper error message if your compiler version is less than the minimum. Previously, it only printed "Could not configure a C++ compiler!" which was not too informative ;)