* Fix unused CMake variable warning
* Acquire msys only on Windows
* Revise APNG variable names and usage
* Fix download mode usability with APNG
* Uncompress APNG patch with gzip
gzip is a default package of MSYS2 and (most) other systems.
7z download is implemented for windows only.
* Revise main CMake patches
cmake.patch deals with libpng build configuration.
fix-export-targets deals with cmake config export.
* Revise pkgconfig patching and fixup
* Additional cleanup
* Add usage based on FindPNG.cmake
* Update versions
Co-authored-by: Leonid Pospelov <pospelovlm@yandex.ru>
