Introduce CMake file linting, based on the cmakelang project by
Josh Bialkowski (@cheshirekow).
Fix various style issues in the CMake files:
* Add the missing copyright header to PNGConfig.cmake.
* Resolve a "missing docstring" warning raised by cmake-lint.
* Rewrite all docstrings in the style of CMake's own documentation.
* Fix whitespace inconsistencies.
References:
* https://pypi.org/project/cmakelang
* https://github.com/cheshirekow/cmake_format
This is a cherry-pick of commit c317fe31114d92d05d44bf98c9b559edf4656b9c
from branch 'libpng18'.
