chore: Increase iOS script portability

[legobeat]

Summary:

pod install and CocoaPods are actually not macOS specific.
Still, the pod lifecycle scripts of react-native depend on macOS-only utilities and will fail on Linux.

This is an attempt to make the scripts portable and make the pod install cleanly on Linux as well as macOS.

Changelog:

[INTERNAL] [FIXED] - Skip XCode patching when not run on macOS
[INTERNAL] [FIXED] - Fall back to `which gcc`/`which g++` to identify C/C++ compiler when `xcrun` not available
[INTERNAL] [FEAT] - Recognize CC and CXX env vars supplied to the script and prefer them over autodetection

Test Plan:

[analysis-bot]

Platform	Engine	Arch	Size (bytes)	Diff
android	hermes	arm64-v8a	19,542,525	+16,370
android	hermes	armeabi-v7a	n/a	--
android	hermes	x86	n/a	--
android	hermes	x86_64	n/a	--
android	jsc	arm64-v8a	22,912,416	+16,352
android	jsc	armeabi-v7a	n/a	--
android	jsc	x86	n/a	--
android	jsc	x86_64	n/a	--

Base commit: 258b481
Branch: main

[NickGerleman]

I kinda suspect this breaks a lot of other assumptions in the build logic. What is use-case and how far does this actually get?

[legobeat]

I kinda suspect this breaks a lot of other assumptions in the build logic. What is use-case and how far does this actually get?

I think the use-case can be summarized as "can run pod install for a project using react-native as dependency on Linux without error". This can be useful e.g. in CI jobs validating Podfile.lock consistency as well as for auditing and experimental development on Linux.

I believe this will retain existing behavior for any scenario that wasn't already erroring before this change.

(Separately, it does feel surprising even on macOS that installing this as a dependency would be automatically and silently be changing the user's Xcode project settings - but that's not the point here)

[cipolleschi]

Changes looks good to me.
I know that cocoapods can be used outside MacOS, but we never encountered the use case before.

[facebook-github-bot]

@cipolleschi has imported this pull request. If you are a Meta employee, you can view this diff on Phabricator.

[legobeat]

Rebased on main; no changes.

[legobeat]

@cipolleschi Thanks!

Some small improvements, should be good to go now I believe. PR description updated.