Allow using system dSFMT

Add USE_SYSTEM_LIBRANDOM variable. Rename librandom to
dSFMT since distributions call it that way.

These changes are required to get Julia packaged in Fedora, since I was not allowed to bundle dSFMT in the Julia package: instead, I'm going to use the system library. This seems to work (i.e. I can get Julia to use the system library), but for some reason `make` always builds and installs dSFMT. Help welcome.

This requires a change in https://github.com/JuliaLang/Rmath/pull/4. I've not tested Rmath at the moment.