Maybe there is a configure option in mysql to put the libs directly in the lib dir instead of a sub directory. Or in zabbix's configure.
Sometimes it's not an explicit option, you need to search the configure scripts to see how the mysql lib is detected.

In the case there is no such option, you can run the binaries with `LD_LIBRARY_PATH=${INSTALL_DIR}/lib:${INSTALL_DIR}/lib/mysql ...` so that the libraries are found properly.