'The \'--version\' option works for clang compilers.
On most platforms, output looks like this::
clang version 3.1 (trunk 149096)
Target: x86_64-unknown-linux-gnu
Thread model: posix
On Mac OS X, it looks like this::
Apple LLVM version 7.0.2 (clang-700.1.81)
Target: x86_64-apple-darwin15.2.0
Thread model: posix'

@classmethod
def default_version(cls, comp):

    if (comp not in cpr._version_cache):
        compiler = Executable(comp)
        output = compiler('--version', output=str, error=str)
        ver = 'unknown'
        match = re.search('^Apple    LLVM    version    ([^    )]+)', output)
        if match:
            ver = (match.group(1) + '-apple')
        else:
            match = re.search('clang    version    ([^    )]+)', output)
            if match:
                ver = match.group(1)
        cpr._version_cache[comp] = ver
    return cpr._version_cache[comp]
