When trying to compile dev-java/jdbc-mysql-3.1.13-r1 without having dev-java/ant-contrib installed, it (reproducably) displays this message (right before the first ant target): [taskdef] Could not load definitions from resource net/sf/antcontrib/antlib.xml. It could not be found. After having installed ant-contrib and re-emerged jdbc-mysql, the message is gone. NOTE: The output of the two builds doesn't differ but for that one line.
Tested this - that warning is harmless, ant-contrib is not required.