FixMacBuild.diff was added with rP24644 to fix an issue with macOS 10.15.7. that prevented spidermonkey from being built.
The number was increased to 11.99 since Apple felt the need to call it macOS 12, the issue is back.
To fix the issue once and for all, it was recommended to simply remove the Max SDK check.
- Bug 1707953 - Remove the max SDK version check on macOS (4/May/21).
The suggested patch in this diff comes from here:
The solution was found by checking the file revisions for toolchain.configure:
I only have access to 10.15.7 and this patch worked for me.