Use https for codegen submodule URI protocol

https is more likely to be allowed through corporate firewalls than the
git protocol
Codegen.py should emit license in amqp_framing.h
Build without amqp_framing.py codegen

Changes to support building rabbitmq-c without having run the python
code generator, or do a git submodule init/update.

This is to facilitate creating links to downloadable source archives
from github