Build: Fixup 428d510f7664. Use check_cxx_source_compiles instead of try_compile
Core/Networking: PoC changes to the way packets are handled

This is a rewrite of the way we send SMSG opcodes, the reasoning behind this is to make fixing packets sent in multiple places easier, and allow for clearer documentation of the packet fields.

Included SMSG_AUTH_RESPONSE and SMSG_AUCTION_COMMAND_RESULT as two examples.