Explicitly set canBecomeKeyWindow on ArgumentsWindow.

In order to make ArgumentsWindow focusable on Mavericks, we need to
explicitly return YES for canBecomeKeyWindow. On older versions of Mac OS X
this worked implicitly: "The NSWindow implementation returns YES if the window
has a title bar or a resize bar, or NO otherwise."
Use compiler-default C dialect to fix SceneKit compilation errors.

C99 dialect sets __STRICT_ANSI__ and messes up the _GLKVector3 layout.
Fix compilation on 10.9 Mavericks.