libpng: Cannot build with Xcode generator

[capnkenny]

pnggroup/libpng#344

Until the patch that was PRed for libpng is in, the new build system for Xcode refuses to build libpng and fails.
Workaround: Only use ninja or Unix Makefiles generators for now.

Fwiw, there's mention that this issue actually appears on all generators now, but Xcode is the only one that flat-out rejects it.

[capnkenny]

The alternative is to host a fork of libpng with the fix cherry picked - this would unblock Xcode generation from failing on libpng. However, zlib would also need evaluation as its CMakeLists.txt file is very similar.

[capnkenny]

Another alternative is that we pull in libpng similar how to this project is doing it: https://github.com/diasurgical/devilutionX/blob/master/3rdParty/libpng/CMakeLists.txt

This appears to build libpng with the ported fix (proposed by the same user) which should unblock this.
@RubyNova - any thoughts or concerns here?

[RubyNova]

If there's a proposed fix that we can reliably target then use that.

Aside from that its up to you.