def prepare_extensions(self):
        """
        Prepare the C{self.extensions} attribute (used by
        L{build_ext.build_ext}) by checking which extensions in
        I{conditionalExtensions} should be built.  In addition, if we are
        building on NT, define the WIN32 macro to 1.
        """
        # always define WIN32 under Windows
        if os.name == 'nt':
            self.define_macros = [("WIN32", 1)]
        else:
            self.define_macros = []

        # On Solaris 10, we need to define the _XOPEN_SOURCE and
        # _XOPEN_SOURCE_EXTENDED macros to build in order to gain access to
        # the msg_control, msg_controllen, and msg_flags members in
        # sendmsg.c. (according to
        # https://stackoverflow.com/questions/1034587).  See the documentation
        # of X/Open CAE in the standards(5) man page of Solaris.
        if sys.platform.startswith('sunos'):
            self.define_macros.append(('_XOPEN_SOURCE', 1))
            self.define_macros.append(('_XOPEN_SOURCE_EXTENDED', 1))

        self.extensions = [
            x for x in self.conditionalExtensions if x.condition(self)
        ]

        for ext in self.extensions:
            ext.define_macros.extend(self.define_macros)

        Prepare the C{self.extensions} attribute (used by
        L{build_ext.build_ext}) by checking which extensions in
        I{conditionalExtensions} should be built.  In addition, if we are
        building on NT, define the WIN32 macro to 1.
        
stackoverflow/venv/lib/python3.6/site-packages/twisted/python/_setup.py