Luckily it is possible to install it:
- Follow the Synology guide for installing ipkg/bootstrapping your NAS. (The DS411j has a Marvel Kirkwood mv6281 ARM processor.)
- Install the required packages with: ipkg install ocaml make gcc
- Now the tricky part, for me Unison didn't compile initially due to errors in /opt/arm-none-linux-gnueabi/lib/libpthread-2.5.so. Somehow this library is not the correct version, but the correct version IS installed in /lib.
Fix this by deleting the symlink /opt/arm-none-linux-gnueabi/lib/libpthread.so.0, and creating a new one to /lib using: ln -s /lib/libpthread.so.0 /opt/arm-none-linux-gnueabi/lib/libpthread/
- Then download the unison source code somewhere (eg /volume1/@tmp), go there and run: make UISTYLE=text NATIVE=false
- There might be some error messages, but in the end a functional unison binary is created!
- Copy the binary to for example /opt/bin. Set up SSH keys for the root account, and we're all set!