Slepc 3.5.2 (update)

Update SLEPc to the newest version.
Add optional dependency on Arpack.
Ah, I see. I don't see any other formula that sets `version` according to the build options. But then I presume you have to install SLEPc twice (once for real and once for complex)?

I'd like to ask @jacknagel to chime in just to make sure this doesn't have unintended side effects. If it looks ok, it seems like a neat way to switch between different versions of PETSc.