Specify a linker library.

        Example:

            LDFLAGS={{ lib("rt") }} {{ lib("pthread", True) }}

        Will unconditionally add `-lrt` and check the environment if the key
        `HAVE_LIBPTHREAD` is set to be true, then add `-lpthread`.

def _lib(self, name, only_if_have=False):
        """Specify a linker library.

        Example:

            LDFLAGS={{ lib("rt") }} {{ lib("pthread", True) }}

        Will unconditionally add `-lrt` and check the environment if the key
        `HAVE_LIBPTHREAD` is set to be true, then add `-lpthread`.
        """
        emit = True
        if only_if_have:
            emit = self.env.get('HAVE_LIB' + self.env_key(name))
        if emit:
            return '-l' + name
        return ''