I found the solution, I write it down so it might help others.
The thing was, that when I shut down my Mac, I updated the Xcode and I forgot this. With that update the DEVELOPER_DIR no longer pointed to /Developer, they removed it and moved everything to /Applications/Xcode.app/Contents/Developer. So the solution is that you move your OgreSDK there, or you can make a symlink into that folder if you don't want to move it. Hope it helps!
Thanks for this hint to masterfalcon at the Ogre forum. (My Ogre forum post with the same issue)