Verified that Window.Close() works as documented on GTK+.
Implemented Window.Close() on Mac OS X, settling both that TODO and the one about making sure Window.Close() works as documented on all platforms.
Resolved NSMakeRect() argument casting TODO in sizing_darwin.m.
Resolved the NSWindowDelegate issue and added an explicit NSApplicationDelegate specifier just to be safe; thanks to Zorg__ in irc.freenode.net/#macdev for explaining (it's an issue with NSApp specifically).
Removed leftover TODO in window_darwin.go.
Re-added Mac OS X versioning macros and compiler/linker options.
Added common_darwin.go; moved toBOOL() there.