Add support for `make LIBFUZZER=YES`. Blocked on Bug 259355 landing first, which adds support for `set-webkit-configuration --[no-]libfuzzer`.
<rdar://problem/112621746>
Pull request: https://github.com/WebKit/WebKit/pull/15954
Committed 266593@main (80eedd698f82): <https://commits.webkit.org/266593@main> Reviewed commits have been landed. Closing PR #15954 and removing active labels.