Hacker News Reader

Moai [1] is very mature, used for a bunch of commercial games, broad support for Mac, Windows, iOS, Android, Chrome (via NaCL), with Linux support forthcoming. Scripted with Lua and/or used directly through C++. Open source.

[1] http://getmoai.com/

Cocos2d-x is another option: http://www.cocos2d-x.org/

You might be interested in Cinder. It's a mature C++ library with multiplatform support: http://libcinder.org/

Cinder is not written for performance. I use it just to open an window and get a basic update/render loop. It is too slow to write games in.

[EDIT] I will actually have a go with polycode to see if it solves the issues I have now with some other projects. Looks good.

with C++ there are a lot of options out there: allegro, sdl, sfml and your favorite text editor.