xcb_auth: Quiet -Wimplicit-fallthrough warning in get_authptr()

xcb_auth.c:135:14: warning: this statement may fall through [-Wimplicit-fallthrough=]
         addr += 12;
         ~~~~~^~~~~
xcb_auth.c:138:5: note: here
     case AF_INET:
     ^~~~

Signed-off-by: Alan Coopersmith <alan.coopersmith@oracle.com>

src/xcb_auth.c

@@ -134,6 +134,7 @@ static Xauth *get_authptr(struct sockaddr *sockname, int display)
         }
         addr += 12;
         /* if v4-mapped, fall through. */
+        XCB_ALLOW_FALLTHRU
 #endif
     case AF_INET:
         if(!addr)

src/xcbint.h

@@ -38,6 +38,16 @@
 #pragma GCC visibility push(hidden)
 #endif
 
+#ifndef __has_attribute
+# define __has_attribute(x) 0  /* Compatibility with older compilers. */
+#endif
+
+#if __has_attribute(fallthrough)
+# define XCB_ALLOW_FALLTHRU __attribute__ ((fallthrough));
+#else
+# define XCB_ALLOW_FALLTHRU /* FALLTHRU */
+#endif
+
 enum workarounds {
     WORKAROUND_NONE,
     WORKAROUND_GLX_GET_FB_CONFIGS_BUG,