Fix the build system

[acw]

The build system is very delicate at the moment, with three major problems:

• Occasionally it won't detect that a file has changed, and thus won't rebuild a library. This seems particularly frequent with HALVMCore and XenDevice files.
• Occasionally it won't rebuild something it should, causing strange crashes. I find this case hard to reproduce, frankly.
• 'make clean' is basically just broken.

I'm not sure if just deleting the current system and starting again or fixing the current system is the better solution, but something should be done.

[acw]

In addition: the Makefile should probably automatically pull down the right versions of the various tools required, because not doing so is causing a lot of problems. In addition, it's hard to count on the underlying distributions having the right things.