make: fix compilation under Mac OS X 10.9 and Xcode 5.0 (closes #30)

The recent updates to OS X 10.9 and Xcode 50.0 broke compatibility with MATLAB default MEX configuration. Furthermore, Xcode 5.0 removes GCC as a supported compiler and clang compiler does not support OpenMP, required for the multi-core features of VLFeat. Compiling VLFeat with OpenMP support on Mac OS X it is still possible by using a GCC installation such as MacPorts. E.g.

$ make CC=/opt/local/bin/gcc-mp-4.7