This should not affect non-Win32 platforms in any way.
As for Win32 (both MinGW and MSVC), this should hopefully allow for
unpatched compilation. Correct functioning is not yet guaranteed though.
1. add the option the code to be used under GPLv2 or GPLv3; previously only GPLv2 was allowed
2. add the "proxy option" as described in GPLv3 so we can add future licenses without having to bulk-edit everything again :)
3. remove the awkward "exception for MIT code in Mac client" clause; it was unnecessary and confusing.