The friendly folks at #musl say:

20:52 < dalias> i think it's good to have both that and the ctor
20:52 < Feh> This should increase robustness if it is in every function.
20:52 < dalias> because...
20:52 < dalias> several of these functions are required to be async-signal-safe
20:53 < dalias> so if the first call happened to be from a signal handler,
   you'd be in trouble if you didn't already have the ctor
20:54 < dalias> in theory another ctor in another lib or the app could run
   before yours and install and run a signal handler :-p but that's pretty
   unlikely and pathological
20:54 < dalias> so i think having both the ctor and the lazy-init keeps you
   safe in situations that will arise in the real world