Fill in/fix macros in WebKitTargetConditionals.xcconfig. Dan Bernstein's multi-version macOS macros were broken/missing over the past few years, so this bug is to track fixing those while adding support for the mysterious future. Note that I removed the multi-version iOS macros in Bug 252076 since they weren't being used. I'm also looking for suggestions on how many older builds for which to keep macros on each OS.
<rdar://problem/123322741>
Pull request: https://github.com/WebKit/WebKit/pull/24827
Committed 275122@main (637fd114532a): <https://commits.webkit.org/275122@main> Reviewed commits have been landed. Closing PR #24827 and removing active labels.