libpng library version hell

[cfriedt]

the libpng library installed by XQuartz should really be the only one that we need. From what I can tell, however, libpng's dylib current and compatibility versioning is all messed up.

Recall, versioning is usually MAJOR . MINOR . PATCH .

Libpng's compatibility version is changed with each new PATCH level increment, which breaks virtually everything. Really, library versions should be based on major.minor, as those should encapsulate all major and minor API changes.

I don't even know what a decent solution to this is. We cannot overwrite XQuartz' libpng, but I also do not want to replace all XQuartz libraries.