add CMake config file that follows CMake conventions

[bebuch]

New functionality:

• add a CMake config file to the install, so an installed libpng can be found via find_package(libpng)
• mark API header includes as SYSTEM headers in install, so warnings in these headers can be disabled

Changed behavior:

• export the target libpng as libpng::libpng which follows the CMake convention for exported targets (previous it was just libpng)
• install CMake files to lib/cmake/libpng instead of lib/libpng to follow the CMake convention

[bebuch]

I have now changed the PR so that it creates the config files I wanted and does not touch anything else. Ready to merge from my side! 😸

[bebuch]

@ctruta If you find some time to work on libpng, it would be nice if you could merge this. That way we can use 1.6.44 without a patch once that is tagged. 🙂

[bebuch]

@ctruta Since I just saw that you are active again, it would be nice if you could merge this ;-)

[ctruta]

@ctruta Since I just saw that you are active again, it would be nice if you could merge this ;-)

My apologies for taking so long, and my appreciations for your patience. This PR is next in line.

[ctruta]

@bebuch Integrated in master, and many thanks. This is super-useful.

Please don't mind me for modifying your commit directly, instead of asking you to do it in a review. After so much wait, now a lot of people want me to push a new release, sooner instead of later. I hope that you accept my modification -- BUT if you do have objections, feel free to submit another PR.