Since PETALSESBCONT-424, slf4j-api and slf4j-jdk14 will be introduced in the system classloader (see comments).
It makes sense to also include jcl-over-slf4j and log4j-over-slf4j that contains classes that replace the commons-logging and log4j classes.
Because slf4j has a very strict policy about changing API, we can be assured that putting them in the system classloader should not introduces bug.
Basically we will use latest version of 1.7.21 that is backward compatible with slf4j api at least since 1.6 (which is very old).