def METHOD_NAME(self):
    """
    Replace sources with .pyx extensions to sources with the target
    language extension. This mechanism allows language authors to supply
    pre-converted sources but to prefer the .pyx sources.
    """
    if _have_cython():
        # the build has Cython, so allow it to compile the .pyx files
        return
    lang = self.language or ''
    target_ext = '.cpp' if lang.lower() == 'c++' else '.c'
    sub = functools.partial(re.sub, '.pyx$', target_ext)
    self.sources = list(map(sub, self.sources))