Remove `add_flags` method in setup.py by fixing cmake config file

[mithro]

The pull request at #48 adds a add_flags method. These flags should be instead correctly set via detection in the cmake file.

The code is shown below;

    def add_flags(self):
        if sys.platform.startswith('win'):
            return


        for flag in ["CFLAGS", "CXXFLAGS"]:
            flags = [os.environ.get(flag, "")]
            if not flags[0]:
                flags.pop(0)


            if shared_options.antlr_runtime == 'static':
                # When linking the ANTLR runtime statically, -fPIC is
                # still necessary because libparse_fasm will be a
                # shared library.
                flags.append("-fPIC")


            # FIXME: These should be in the cmake config file?
            # Disable excessive warnings currently in ANTLR runtime.
            # warning: type attributes ignored after type is already defined
            # `class ANTLR4CPP_PUBLIC ATN;`
            flags.append('-Wno-attributes')


            # Lots of implicit fallthroughs.
            # flags.append('-Wimplicit-fallthrough=0')


            if flags:
                os.environ[flag] = " ".join(flags)