Benefits:
- newer versions of the libraries
- zlib: 1.2.8 -> 1.2.11
- libpng: 1.6.18beta04 -> 1.6.34
- much fewer dependencies to install (as libs are already in optipng archive and we don't need napa)
- much smaller build thanks to customised versions of zlib and libpng containing only APIs necessary for optipng itself: 238950 -> 177359 bytes
- much faster build thanks to preconfigured libpng and stripped APIs: 2m15s -> 40s
- much simpler build script: 77 -> 46 lines