Public domain does not have an internationally agreed upon definition,
As such it's generally preferred to use an extremely liberal license,
which can explicitly list the rights granted by the copyright holder.
The CC0 license is the usual choice here.
This "relicensing" is done without hunting down copyright holders, since
it is presumed that their release of this work into the public domain
authorizes us to redistribute this code under any other license of our
choosing.
JosJuice