cmake: Enable the libpng framework build by default on Apple systems

The PNG_FRAMEWORK option used to be off by default. It was possible to
turn it on, regardless of the underlying operating system, but doing so
outside of an Apple OS broke the libpng build.

PNG_FRAMEWORK is now on by default, conditionally defined on Apple
systems only, and it is ignored (without breaking the build) elsewhere.

Other minor changes have also been applied.
1 file changed