utils.h: add fallthrough macro

This can be used to silence clang warnings about unannotated fall-through

Signed-off-by: Felix Fietkau <nbd@nbd.name>

utils.h

@@ -227,6 +227,18 @@ int clock_gettime(int type, struct timespec *tv);
 #define __hidden __attribute__((visibility("hidden")))
 #endif
 
+#ifndef __has_attribute
+# define __has_attribute(x) 0
+#endif
+
+#ifndef fallthrough
+# if __has_attribute(__fallthrough__)
+#  define fallthrough __attribute__((__fallthrough__))
+# else
+#  define fallthrough do {} while (0)  /* fallthrough */
+# endif
+#endif
+
 int b64_encode(const void *src, size_t src_len,
 	       void *dest, size_t dest_len);