Update to 0.5.5.

This is mostly a bug fix release. Change log:
https://github.com/pngwriter/pngwriter/blob/283c34deaf775d0a03426fe082de1681c16d136f/CHANGELOG.md

Thankfully the port now has a CMake-based build system, so we can drop a lot
of hand-made build and installation code from the Makefile.