Fiddle with the makefile (disable xfbuild)
Allow multiple RNGs for a neuron group.

Access via 'rand#()' and 'randn#()'. rand() and randn() are same as rand0() and
randn0(). By default, there are no RNGs and I've disabled the check for that
for simplicity... it'll now generate a vanilla compillation check instead. Had
to change the API of the Seed method of INeuronGroup.