We fail in the install headers step using an alternate root. There are two issues: There are extraneous ')' at the end of two NORMAL_PRODUCTION_FRAMEWORKS_DIR_... rules. The computation of OUTPUT_ALTERNATE_ROOT_PATH for macOS should produce a null value.
<rdar://86795488>
Created attachment 447780 [details] Patch
Committed r287361 (?): <https://commits.webkit.org/r287361>