/usr/lib/compat contains a bunch of "compatibility" versions of certain libraries. These are used by certain PortMaster ports, such as 'Deltarune'.
This path should be added to LD_LIBRARY_PATH in control.txt. ROCKNIX's control.txt or PlumOS's control.txt may provide some details here, but the current versions of either of these are pretty different from RogueOS's control.txt. Edit: I see now that past versions of RogueOS's control.txt once more closely resembled these, and did contain the export in question.
A user may edit the version of this file located at /storage/roms/ports/PortMaster/control.txt and add such an export themselves—which will fix affected ports—but the change will be destroyed each time PortMaster is launched.