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

[ctruta]

The code looks good overall -- but see my comment about the libpng vs. PNG namespace.

I would also like to have one commit per one task, submitted on top of the tree, as a general rule. We are sticking to a linear history, going forward, and we reserve the branches for the time when we fork new libpng versions. Could you please squash all commits, then rebase?

Last, if you could please update scripts/cmake/AUTHORS.md 🙂

[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.