-Wimplicit-fallthrough=3 is added by -Wextra, which we already set.
Bumping the warning level only changes how fall-through comments are
parsed, with level 5 turning off fall-through comments entirely.
This is unnecessary and results in having to do extra work to squash
warnings when included external code uses fall-through.
This change also adds /* FALLTHROUGH */ comments where
H5_ATTR_FALLTHROUGH is used so compilers that don't use attributes
but do respect fall-through comments don't raise spurious warnings.
