Annotate intentional handler fallthrough

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

include/akerror.tmpl.h

@@ -391,6 +391,18 @@ akerr_ErrorContext AKERR_NOIGNORE *__akerr_copy_string(char *destination, int ca
  * Defines for the ATTEMPT/CATCH/CLEANUP/PROCESS/HANDLE/FINISH process
  */
 
+/*
+ * HANDLE_GROUP deliberately enters the next case label. GCC and clang both
+ * understand this spelling in the C modes supported by libakerror. Other
+ * compilers keep the established control flow without receiving an attribute
+ * they do not implement.
+ */
+#if defined(__GNUC__) || defined(__clang__)
+#define AKERR_FALLTHROUGH __attribute__((fallthrough));
+#else
+#define AKERR_FALLTHROUGH
+#endif
+
 #define ATTEMPT					\
     switch ( 0 ) {				\
     case 0:					\
@@ -443,6 +455,7 @@ akerr_ErrorContext AKERR_NOIGNORE *__akerr_copy_string(char *destination, int ca
     __err_context->handled = true;
 
 #define HANDLE_GROUP(__err_context, __err_status)			\
+    AKERR_FALLTHROUGH					\
     case __err_status:							\
     __err_context->stacktracebufptr = (char *)&__err_context->stacktracebuf; \
     __err_context->handled = true;