The ctl part of rlib has various memory leaks (ref https://www.riscosopen.org/forum/forums/4/topics/9503) reflecting its abandoned state. See the FrontEnd module for a more flexible implementation of the same GUI-in-a-text-file scripting language.
The ctl API was not in C release 3 (it post dates that) nor in C release 4 manual, and the whole of rlib was removed from C release 5 in favour of the Toolbox.
The change log at the top of rlib/s/rl_spare shows it was briefly added in early 1990 then removed in April 1991. 26 years later, cull it from the Makefile too.
Version 5.94. Tagged as 'RISC_OSLib-5_94'