RT mode and 32/64/FAT added

This adds a --realtime-libs option which enables the installation of a "lightweight" version of Essentia with everything omitted except for FFTW. It outputs the library name "libessentia_rt.a" so the user can have the full version installed also.

Another option --arch=i386, x86_64, FAT allows Mac users to install the 32-bit/64-bit and universal versions of the library provided the dependencies are appropriate. This is useful as some applications like Pd-Extended and plugin formats like VST still exist as 32-bit.