expose amqp_login with va_list as args

fix invalid Cflags property name

rabbitmq-c v0.11.0

rabbitmq-c v0.10.0

rabbitmq-c v0.9.0

This is exactly the same as the v0.9.0; but associated with the same
commit on the master branch. See:
alanxz#524

rabbitmq-c v0.9.0

rabbitmq-c v0.8.0

rabbitmq-c v0.7.1