R2021a+: Dynamic selection of slf4j/log4j libs

[apjanke]

Once I've got packaged-munged JARs of slf4j and log4j 1.x built, the M-code layer will need to dynamically select between them based on the Matlab version it's running under.

Need to be careful with how this is done: Static references to nonexistent Java classes can break the build under Matlab Compiler, so we may need to use reflection mechanisms for this.

Obviously, make it Go Fast.

Blocks #15.
Blocked by janklab/slf4m-jankalog#4.