The contents of the library build out directory, LIBODIR, are being installed to /opt/synchronet/exec. The second path noted on line 116 in src/sbbs3/GNUmakefile can be changed to $(SBBSDIR)/exec like it is for the first path in order for it to come up that way. Might be best, for now, to make (& document) the change in the original source archive.
Added & corrected note to debian/README.source about correcting the LD_RUN_PATH line in src/sbbs3/GNUmakefile. (r164)(r165)