This happens because MATLAB 2015b suports only Xcode 5.1+ or 6.x. However, with the recent upgrade of Xcode to 7.0, it became unrecognizable for MATLAB. You can try the following:
1. Go to preferences directory. You can get the path to this folder by executing:
on the MATLAB command prompt.
2. Ensure that there is no mex_*.xml file present in this folder, delete if there are any.
3. Replace the MEX option files present in "MATLABROOT/bin/maci64/mexopts" directory with the ones attached in a zip file( "files.zip") with this post.
4. Restart the MATLAB.
Hope this helps