I'm writing an application for which I want to use some 20-year old open source code. The original code is kind of antiquated in certain aspects, for example, it's written in K&R C rather than C99; it has to manually compute certain things that are now defined in C standard library headers; there are no tests; and so on and so forth. All that the original code says by way of licensing is "Released in the public domain by so and so"; no BSD/MIT/GPL, just "public domain".
In addition to my original application, I'd like to release my own fork of the legacy library with a few of changes, chiefly to modernize it and to add unit tests. I've contacted the original author about it, but have not heard back from him. Am I within my rights to release my fork with a liberal (BSD/MIT) license? I'm not trying to make any money off of either my update of the legacy library or off of the application built on top of it.