Better fallthrough annotation macro.

src/include/86box/plat_fallthrough.h

@@ -18,8 +18,13 @@
 #ifndef EMU_PLAT_FALLTHROUGH_H
 #define EMU_PLAT_FALLTHROUGH_H
 
-#if !defined (__APPLE__) && !defined(__clang__)
-#    define FALLTHROUGH_ANNOTATION
+#if __has_attribute(__fallthrough__)
+# define fallthrough __attribute__((__fallthrough__))
+#else
+# if __has_attribute(fallthrough)
+#  define fallthrough __attribute__((fallthrough))
+# endif
+# define fallthrough do {} while (0) /* fallthrough */
 #endif
 
 #endif /*EMU_PLAT_FALLTHROUGH_H*/