Unify more of xcconfig content across projects

• Component: WebKit Misc.
• Status: RESOLVED FIXED

[Alexey Proskuryakov]

Move more common variables from individual projects' xcconfigs to a central location, refactoring where they diverged.

[Radar WebKit Bug Importer]

<rdar://problem/103600264>

[Alexey Proskuryakov]

Pull request: https://github.com/WebKit/WebKit/pull/7958

[Alexey Proskuryakov]

The configs are still far from what I'd consider nicely organized. My goal here was to move as much as possible, reducing the amount of almost-duplicated code with subtle differences. This will make it possible to have verifiably correct PRs for further cleanup.

Some TODOs that I wrote down while working on this:

- Update TARGET_MAC_OS_X_VERSION_MAJOR (and delete TARGET_MACOS_LEGACY_VERSION_IDENTIFIER)

- Clean up SEARCH_PATHS, they generally make no sense
    - split them by production (deployment_location?) vs. engineering 

- Is SUPPORTED_PLATFORMS needed anywhere?

- Eliminate WK_WHICH_BUILD_SYSTEM

- DumpRenderTree.app - skip install on macOS (unsure about Catalyst)

- Normalize prefixing search paths with SDK_DIR/SDKROOT (should be consistent, and maybe neither is needed)
    - Full path is currently needed when makefiles use the variable

- Normalize when we use $(inherited), and when we don't.

- _PATH vs. _DIR naming - which are full file system paths? Which ones end with a slash? Should we have something like _IF_NEEDED on those that are empty when unused?

- What is COPY_STAGED_FRAMEWORKS_TO_SECONDARY_PATH? Why do we ever need to copy during build?

- Use actual names for cryptexes etc. in variables, not the confusing "override/alternative" naming.

- libANGLE-shared.dylib and libwebrtc.dylib LC_ID_DYLIB's are inconsistent in STP (one is loader path relative, another is absolute).

[EWS]

Committed 258266@main (e31966cc25b1): <https://commits.webkit.org/258266@main>

Reviewed commits have been landed. Closing PR #7958 and removing active labels.