Avoiding or speeding up full tree recompilations

[Jordi Gutiérrez Hermoso]

Whenever someone touches any build files, I think, a full tree
recompilation ensues. E.g. touching foo.mk in order to add some bar.m
seems to trigger this.

In the interest of economising CPU cycles and slowing down the heat
death of the universe, can this be helped? Perhaps ccache could be
used in order to avoid recompiling at least a few things regularly
while working with the development sources.

I ask because I don't know if it can be avoided. I've had a few
half-hearted attempts to understand Octave's build process, and maybe
I should try again, but I'd rather ask you people first.

Thanks,
- Jordi G. H.