Fixed compilation for MacOS with Xcode 5.

Made several simple fixes in order to get all examples built on OS X 10.9.4 with Xcode 5.1.1

1. Unfortunately [you can't](http://industriousone.com/debugdir) set working directory for an Xcode project from premake configuration file, you have to do it manualy.
2. For reasons unknown [they moved](http://stackoverflow.com/a/5929887/1682275) `<malloc.h>` to `<malloc/malloc.h>`.
3. Forcing architecture type (x86/x64) is undesirable for Xcode. Xcode tries to select it automatically. If architecture is forced in the generated Xcode project, you have to select it manually because symbols (e.g. Cocoa) might be unavailable for particular architecture when linking. Making a *Universal* build prevents from manual choosing of architecture.
4. They moved `<gl/glext.h>` to `< OpenGl/glext.h>` as well.
5. Some GL defines were missing.
6. `APIENTRY` is not defined on not-Windows platforms.

P.S. Please replace **start** with **open** in my README.md changes (damn copy-pasting):
`open .build/projects/xcode4/bgfx.xcworkspace`