Fix compilation issues on iOS platform

[nmcc24]

Resolves #1015

Short description 📝

Currently, XcodeProj is not compilable on iOS platform. This is an issue when attempting to compile products that have XcodeProj as a (transitive) dependency

Solution 📦

Add appropriate iOS compiler flag and version check to ensure symbols are available and guarantee successful compilation.

Implementation 👩‍💻👨‍💻

Detail in a checklist the steps that you took to implement the PR.

• Add compiler flags
• Add iOS version checks
• Build XcodeProj target
• Compile XcodeProj tests

[nmcc24]

@fortmarek @cschmatzler @asmitbm could you please take a look at this and let me know your thoughts? Thanks a lot

[pepicrft]

@nmcc24 this package was not designed for running on non-macOS platforms, so even if you make it compile, some features might not work. I'd suggest to use conditional linking when declaring a dependency on this package to specify that it should only link it when building against the macOS platform.