Add iOS headless building support

[slingthor]

Signed-off-by: Ingthor Hjalmarsson [email protected]

[ctruta]

Hello, and thank you very much for your contribution.

Although I agree with your idea in principle, I wonder if you could please refactor it in a way that does not complicate the current use of add_executable. Would it not be easier (both to implement and to understand) if you could please just surround the option definitions PNG_TESTS and PNG_TOOLS with: if(IOS) then off by default (possibly with a brief comment as to why they're off by default on iOS) else() on by default, endif() ...?

I'm open to other simplifying ideas as well.