Fix not being able to checkout the mruby submodule.

I locally fixed a C++ problem, it’s now in upstream
(https://github.com/mruby/mruby/issues/1019). It should now checkout
properly again (I updated the mruby subdmodule to the current upstream
master).
Add libintl dependency and properly detect CEGUI (hopefully).
We only need libintl on Windows, it seems