[jamming] Undesired multi pass build

[Arnt Gulbrandsen]

That can't be done, I'm afraid. But maybe the problem can be restated so 
that the goal is achievable.

Jam's design is (simplifying):

1. Look at the files.
2. Decide what to do.
3. Do it.

Nice and simple. But it prevents you from e.g. first generating common.h 
and then deciding whether to compile source.o.

So, what can you do? One possibility is to tell jam explicitly that 
source.o depends on common.h, so that whenever jam decides to build 
common.h, it will also build source.o.

Arnt