Upstream the definition of HAVE_READ_ONLY_SYSTEM_VOLUME
Created attachment 394019 [details] Patch
Makes sense to me. Thanks for being on top of this.
Committed r258737: <https://trac.webkit.org/changeset/258737> All reviewed patches have been landed. Closing bug and clearing flags on attachment 394019 [details].
<rdar://problem/60657258>