look for libdiscid.0.dylib

This is the file name provided in the official Mac distribution of libdiscid
and a library in the current folder will be found.
A system-wide install is also found using this name, but not without it.

I somehow wasn't allowed to attach the commit to #6.
Not sure what the exact requirement is. I could do that to repositories without commit access before. Possibly only when I created the issue.