[IMP-dev] building imp faster on laptops

You can greatly accelerate building IMP on your mac laptop (and reduce the chance of it saturating you I/O bus and making your machine temporarily useless) by sticking the build directory in a ram disk. The preferences panel "Esperance DV" helps make such a ram disk (and to save the contents back to disk when you log out). You need a ram disk of at least 1.2GB. And the ram disk seems to get paged to disk OK when I'm doing other things on my mac (so it is not permanently eating the memory).