build: Fix various compiler warnings

This fixes a few compiler warnings that were encountered with gcc 9.3.0 and musl libc 1.1.24 during the autotools build.

Many of the warnings addressed are fallthrough warnings for switch statements. While this could be addressed with compiler flags, such a solution would run the risk of missing behavior like that remedied in !72 (merged). However, the solution given here of fallthrough comments is quite verbose in some places.

Thank you in advance for any reply.