build-sys: Enable composefs at *build time* by default

build-sys: Enable composefs at *build time* by default

There's no additional dependencies, and it's a small amount
of new code.

The riskiest thing is the changes to ostree-prepare-root, but
I believe that things are in a good state now there.

Again, this just enables it at *build time* - it's still
off at runtime by default.