Fix unused platform check and configuration for macOS

[zixu-w]

In a similar manner as zlib (https://github.com/madler/zlib/pull/895), libpng contains a header configuration that's no longer valid and hasn't been exercised for the macOS target.

• The target OS conditional macros are misused. Specifically TARGET_OS_MAC covers all Apple targets, including iOS, and it should not be checked with #if defined as they would always be defined (to either 1 or 0) on Apple platforms.
• #include <fp.h> no longer works for the macOS target and results in a compilation failure. macOS ships all required functions in math.h, and clients should use math.h instead.

This problem has not been noticed until a recent extension in clang (https://github.com/llvm/llvm-project/pull/74676) exposed the issue and broke libpng builds on Apple platforms. The failure can be reproduced now by adding #include <TargetConditionals.h> before the block.

[ctruta]

Thank you very much for making the kind of contribution that I personally like the most: deleting code that should no longer be in there 😛

I landed your commit with your name added to the AUTHORS file, but may I please kindly ask for the native spelling of your name as well? Although that file is in ASCII right now, I want to convert it to Markdown and Unicode, eventually.

[zixu-w]

Thanks for the quick review!

FYI: 王子旭