hexchat: 2.12.3 -> 2.12.4

- Update hexchat to version 2.12.4
- Use sources from github, as the source at
http://dl.hexchat.net/hexchat/hexchat-2.12.4.tar.xz contains invalid symbolic
links in the directory 'build-aux'
- Run 'autogen.sh' to generate 'configure' and friends, as they are not
available in the distributed sources from github.