These were intended originally to allow a hybrid build system: you
could build an e3 module as the dynamically loaded shared library,
but you could also run `make epics` and build it as a vanilla EPICS
module.

However, this has never really worked, nor has it been supported. It
may in the future be supplanted by properly using the EPICS build
rules in lieu of the use of driver.makefile, but that is for a future
change.