Try to make minetest build with cmake on osx.

Stuff kind of works. What's missing:
- Copy Irrlicht.framework to app bundle
- Stuff the compiled MainMenu.nib into .app
Document current state. Read this commit message!

What's still wrong:

1) Put the Irrlicht.framework in the app bundle

It has to end up in
`Contents/Resources/Frameworks/Irrlicht.framework`

2) Put the compiled MainMenu.nib in the app bundle

After the build it resides in
`bin/Debug/Contents/Resources` but it should be in
the final app bundle as
`Contents/MainMenu.nib`

Conflicts:
	README.txt