Build: Use -Wimplicit-fallthrough=5 when supported

Now that we have the FALLTHROUGH macro, use the strictest mode with
GCC so that comment-based fallthrough markings are no longer accepted.

In GCC, -Wextra includes -Wimplicit-fallthrough=3 and
-Wimplicit-fallthrough is the same as -Wimplicit-fallthrough=3.
Thus, the strict mode requires specifying -Wimplicit-fallthrough=5.

Clang has -Wimplicit-fallthrough which is *not* enabled by -Wextra.
Clang doesn't have a variant that takes an argument. Thus we need
to check for -Wimplicit-fallthrough. Do it before checking for
-Wimplicit-fallthrough=5 so that the latter overrides the former
when using GCC.

CMakeLists.txt

@@ -434,6 +434,8 @@ if(CMAKE_C_COMPILER_ID MATCHES GNU|Clang)
                 -Wmissing-prototypes
                 -Wmissing-declarations
                 -Wredundant-decls
+                -Wimplicit-fallthrough
+                -Wimplicit-fallthrough=5
 
                 -Wc99-compat
                 -Wc11-extensions

configure.ac

@@ -1299,6 +1299,8 @@ AS_IF([test "$GCC" = yes], [
 			-Wmissing-prototypes \
 			-Wmissing-declarations \
 			-Wredundant-decls \
+			-Wimplicit-fallthrough \
+			-Wimplicit-fallthrough=5 \
 			\
 			-Wc99-compat \
 			-Wc11-extensions \