Bug fix: make.opt.prefix works now.
I'd set make.opt.prefix's default to "~my" rather than "~/my", so I've
corrected that. But I've also updated the default (in make.py) to None.
This leaves it to the calling code to set up make.opt.prefix if it
hasn't been set. There's now a boiler-plate section at the top of make
that handles this.