I just ran into the same problem on Debian 8.7.
TL;DR: copy the compiled library file to a directory on the load path.
The details
I found where to copy it by looking at where the others were loaded from like this:
ldd $(which afp_client)
Which should show lines like this:
libpthread.so.0 => /lib/x86_64-linux-gnu/libpthread.so.0 (0x00007f9b601e2000)
The missing library should have been built by the make command so I searched for it in the afpfs-ng directory like so:
find . -name libafpclient.so.0
Which was found here:
./lib/.libs/libafpclient.so.0
So I copied it like this:
cp lib/.libs/libafpclient.so.0 /lib/x86_64-linux-gnu/
There might be a more proper location for this to be copied to, but I'm working on a throw away VM and this made it work, so I didn't worry about it.
Update:
I believe the proper place to put this would be /usr/local/lib/.
