I am having the same problem on a macbook air. Since I upgraded to El Capitan I can't run my mysql apps.
What did you do to change " the shared library identifier of libmysqlclient.18.dylib so that dyld looks for the binary relative to my application binary"?
It seems the upgrade removed the lib from /usr/lib, so I downloaded the latest build (10.8 osx and v6.1.6) and linked the library into /usr/lib. Initially, I got an error saying it could not connect to the socket /tmp/mysql.sock, so I fixed that by changing XAMMP configuration and that seems okay.
I now just get BAD_ACCESS when running the app, but no diagnostic of any use.
a bit late, but hpe can help
1) ln ..
does not work any more in Catalina
2) mysql lib are newer.. with less bugs but as always bad....