configure.py: make build.ninja the same every time
configure.py: make build.ninja the same every time

In several places, configure.py uses unsorted sets which results in
its output being in different order every time - both a different
order of targets, and a different order in dependencies of each
target.

This is both strange, and annoying when trying to debug configure.py
and trying to understand when, if at all, its output changes.

So in this patch, we use "sorted(...)" in the right places that
are needed to guarantee a fixed order. This fixed order is alphabetical,
but that's not the goal of this patch - the goal is to ensure a fixed
order.

Signed-off-by: Nadav Har'El <625169824b719df98706d88646d0aae6f368ae4d@scylladb.com>

scylladb/scylla,scylladb/scylla,scylladb/scylla,scylladb/scylla