I don't want to complicate build_all too much, since it's intended to be just a
small convenience, and I still hope that we can eventually migrate to a better
build tool (see issue67).

What we could easily do is change build_all so that it forwards all arguments to
the make calls it invokes. Cleaning up everything could then be performed with:

$ ./build_all distclean

followed by

$ ./build_all

To use 4 parallel build processes, do something like:

$ ./build_all -j4

Does this address the wish?

(One small wrinkle ist that "./build_all distclean" will complain at the end
that "validate" cannot be found. But I think we can ignore this.)

While we're adding command-line options to build-all, adding support for
additional options to be passed on to make would be very useful. In particular,
I often wish that I could call build-all with -j4 or even -j8 to speed it up.

For some cases (e.g., after a big pull from upstream) it might be an improvement
to have build_all not simply use make in each of the src/ subdirectories but
first do a 'make clean', as well.

Perhaps a command-line argument such as -fresh could be used for the purpose.