On rolling distros the usage of `libpng` submodule makes problems as the
`zlib` system dependency of said module gets updated. Then the submodule
regularly is too old to handle the updates `zlib` dependency.
Fix this maintenance churn by requiring `libpng` as system library as
well. Then the distros package manager keeps those versions in sync.
This unfortunately requires users to install a new runtime/build
dependency, but it still can be disabled with `-DWITH_PNG=OFF`.
f4b729e00b