The order of elements in sets cannot be relied upon, because the hash
values are randomized by Python. Whenever sets are converted to lists
we need to keep their order stable, or random changes in the command line
cause ninja to rebuild a lot of files unnecessarily. To stabilize them,
use either sort or OrderedSet. Sorting is not always applicable, but it
can be faster because it's done in C and it can produce slightly nicer
output.
In qemu, minikconf generates a depfile that meson could use to
automatically reconfigure on dependency change.
Note: someone clever can perhaps find a way to express this with a
ninja rule & depfile=. I didn't manage, so I wrote a simple depfile
parser.
Paolo Bonzini