I have just succeeded on building it. Admittedly, it didn’t work flawlessly out of the box – it failed when building with the sandbox. Which makes me wonder if this bug is related to the sandbox (i.e., overlayfs). Could you try this?
mkdir -p install_dir
make install DESTDIR=$PWD/install_dir
Let me know if this works. If it does, then it confirms we have a bug in the unionfs implementation.
Alternatively, you could try to edit
/System/Settings/Scripts/Directories.conf and move
unionfs-fuse to the top of the
Compile Glibc again and see if things go different this time.