Return the -std=c++[0x/11/14] compiler flag.
    The c++14 is preferred over c++0x/11 (when it is available).

def cpp_flag(compiler):
    """Return the -std=c++[0x/11/14] compiler flag.
    The c++14 is preferred over c++0x/11 (when it is available).
    """
    standards = ['-std=c++14', '-std=c++11', '-std=c++0x']
    for standard in standards:
        if has_flag(compiler, [standard]):
            return standard
    raise RuntimeError(
        'Unsupported compiler -- at least C++0x support '
        'is needed!'
    )