Hi Graham, I was able to get tarsnap to compile. I'll update what I did for others who may find themselves in the same situation.
My system is Synology DS414. It has the arm7l CPU. I needed to do the following to get everything to work: 1. I installed entware-ng which provides a development environment. See here: https://github.com/Entware-ng/Entware-ng/wiki/Install-on-Synology-NAS 2. I then installed gcc. See here: https://github.com/Entware-ng/Entware-ng/wiki/Using-gcc-(native-compilation) 3. I installed development headers following the instructions in the above link. The problem is that entware-ng doesn't create some of the lib symlinks properly creating configure problems when compiling tarsnap. So you need to use "opkg install --force-reinstall" to reinstall openssl and other packages configure complains about. After, compiling is simple and everything works. Thank you to everyone who tried to help. Arun.
