Redo fixes

[apenwarr]

This fixes some bugs and inefficiencies in the redo-based build scripts. In particular, using the redo from https://github.com/apenwarr/redo, we now get a substantial benefit from using 'redo -j10' on my machine.

The initial build is still slower with redo than with make, because:

• default.o.do uses -O3 while the Makefile is not

• default.o.do is much more careful (too careful? :)) about file dependencies, while the Makefile just has .o depend on .h.

But at least it's much faster than before.

[linleyh]

To be honest, I don't understand redo (I didn't write those scripts) but I'll trust you!