Older gcc compilers generate a warning when the attribute is used.
I found out that GCC 7 will not produce a warning if comments
such as /* fall-thru */ or /* FALLTHRU */ are used instead of the
attribute [[fallthrough]]
"-Wextra" option turns on the following warnings:
-Wclobbered
-Wempty-body
-Wignored-qualifiers
-Wmissing-field-initializers
-Wmissing-parameter-type (C only)
-Wold-style-declaration (C only)
-Woverride-init
-Wsign-compare
-Wtype-limits
-Wuninitialized
-Wunused-parameter (only with -Wunused or -Wall)
-Wunused-but-set-parameter (only with -Wunused or -Wall)
