libpng is likely to bring sorrow and regret. I strongly recommend to use a native Rust PNG library instead (e.g. LodePNG).
This crate bundles libpng 1.6.44 as a fallback. Please check whether this version is still secure before using this package.
If you add it as a build dependency, Cargo will make DEP_PNG_INCLUDE env var available, containing a path to a directory with png.h.
You may need use libpng_sys as _; in your Rust program to ensure it's "used" and actually linked.