Fix Arm Neon optimization detection when building for iOS

[TheMostDiligent]

When cross-compiling for iOS, CMake does not define the CMAKE_OSX_ARCHITECTURES variable. As a result, this branch of CMake script does not get executed, so PNG_ARM_NEON_OPT is not defined as well as source files for Neon are not added to the project.

The PNG_ARM_NEON_OPT then gets defined here which results in linker errors as source files are not in the project.

[TheMostDiligent]

Is it possible that you merge this minor change? It fixes link errors in XCode