Annotate intentional handler fallthrough

Co-authored-by: Andrew Kesterson <andrew@aklabs.net>

CMakeLists.txt

@@ -263,6 +263,14 @@ foreach(_test IN LISTS AKERR_TESTS)
     endif()
 endforeach()
 
+# HANDLE_GROUP deliberately enters the next case label. Keep that public macro
+# compiling with the warning enabled, so a future macro edit cannot restore the
+# warning for consumers which adopt -Wextra.
+if(CMAKE_C_COMPILER_ID MATCHES "GNU|Clang")
+    target_compile_options(test_err_handle_group PRIVATE
+                           -Werror=implicit-fallthrough)
+endif()
+
 set_tests_properties(
     ${AKERR_WILL_FAIL_TESTS}
     PROPERTIES WILL_FAIL TRUE