Better fix for #40241 (--enable-all-bugs-are-fatal and fallthrough)
This one should work on GCC and on Clang. The previous version made Clang happier by not having unreachable "fallthrough" statements, but made GCC sad because GCC didn't think that the unconditional failures were really unconditional, and therefore wanted a FALLTHROUGH.
This patch adds a FALLTHROUGH_UNLESS_ALL_BUGS_ARE_FATAL macro that seems to please both GCC and Clang in this case: ordinarily it is a FALLTHROUGH, but when ALL_BUGS_ARE_FATAL is defined, it's an abort().
Fixes bug 40241 again. Bugfix on earlier fix for 40241, which was merged into maint-0.3.5 and forward, and released in 0.4.5.3-rc.