Add support for Darwin "bundles" (loadable modules)

This is a little confusing, as there are two definitions of bundles.

The first is a structured directory. Frameworks are bundles, apps are bundles, and bundles[2] are... bundles[1].

The second is a loadable module. Similar to a shared library, but you can't actually link against it (results in a linker error) and instead must load it through the dyld APIs. This is primarily used for developing plugins (and later when Qt itself is based on Qbs it should use these for Qt plugins instead of dylibs!).

We have some support for the first kind of bundle, but not the second. The linker argument is -bundle rather than -dynamiclib or -static.

Let's refer to what Mach-O calls "bundles" as "loadable modules" and "structured directories" as bundles.

Xcode 5 can build bundles out of the box on iOS (notably it CANNOT build dylibs or frameworks unless you add the xcspecs (which is trivial)). My guess is that this has no bearing on bundles being supported prior to iOS 8, and they merely left the xcspec there in order to support unit testing, which uses bundles.

Some info: http://docstore.mik.ua/orelly/unix3/mac/ch05_03.htm