Now mex in MATLAB 2012a only officially supports gcc 4.4.6 but I want to use gcc 4.7 at my own risk. Now If I compile something with mex directly, it will complain that
/usr/lib/gcc/i686-linux-gnu/4.7/cc1plus: /usr/local/MATLAB/R2012a/sys/os/glnx86/libstdc++.so.6: version `GLIBCXX_3.4.15' not found
The problem is that when you are building with mex, it puts -L/usr/local/MATLAB/R2012a/sys/os/glnx86 on the link line, and so the linker picks up libstdc++.so from there.
If you can't convince mex to prepend -L/usr/lib/i386-linux-gnu first, then I think your only other choice is to remove /usr/local/MATLAB/R2012a/sys/os/glnx86/libstdc++.so (just rename it to e.g. libstdc++.so.bak).