macros: Add G_GNUC_FALLTHROUGH

Expands to the GNU C fallthrough statement attribute if the compiler is gcc.
This allows declaring case statement to explicitly fall through in switch
statements. To enable this feature, use -Wimplicit-fallthrough during
compilation.

docs/reference/glib/glib-sections.txt

@@ -438,6 +438,7 @@ G_GNUC_DEPRECATED_FOR
 G_GNUC_BEGIN_IGNORE_DEPRECATIONS
 G_GNUC_END_IGNORE_DEPRECATIONS
 G_GNUC_NORETURN
+G_GNUC_FALLTHROUGH
 G_GNUC_UNUSED
 G_GNUC_PRINTF
 G_GNUC_SCANF

glib/docs.c

@@ -2280,6 +2280,22 @@
  * See the GNU C documentation for more details.
  */
 
+/**
+ * G_GNUC_FALLTHROUGH:
+ *
+ * Expands to the GNU C fallthrough statement attribute if the compiler is gcc.
+ * This allows declaring case statement to explicitly fall through in switch
+ * statements. To enable this feature, use -Wimplicit-fallthrough during
+ * compilation.
+ *
+ * Put the attribute right before the case statement you want to fall through
+ * to.
+ *
+ * See the GNU C documentation for more details.
+ *
+ * Since: 2.60
+ */
+
 /**
  * G_GNUC_UNUSED:
  *

glib/gmacros.h

@@ -191,6 +191,12 @@
 #define G_GNUC_NO_INSTRUMENT
 #endif  /* !__GNUC__ */
 
+#if    __GNUC__ > 6
+#define G_GNUC_FALLTHROUGH __attribute__((fallthrough))
+#else
+#define G_GNUC_FALLTHROUGH
+#endif /* __GNUC__ */
+
 #if    __GNUC__ > 3 || (__GNUC__ == 3 && __GNUC_MINOR__ >= 1)
 #define G_GNUC_DEPRECATED __attribute__((__deprecated__))
 #else