Add a fallthrough macro.

This macro defers to __attribute__((fallthrough)) on GCC (and
clang).  Previously we had been using GCC's magic /* fallthrough */
comments, but clang very sensibly doesn't accept those.

Since not all compiler recognize it, we only define it when our
configure script detects that it works.

Part of a fix for 34078.

src/lib/cc/compat_compiler.h

@@ -50,6 +50,12 @@
 #define CHECK_SCANF(formatIdx, firstArg)
 #endif /* defined(__GNUC__) */
 
+#if defined(HAVE_ATTR_FALLTHROUGH)
+#define FALLTHROUGH __attribute__((fallthrough))
+#else
+#define FALLTHROUGH
+#endif
+
 /* What GCC do we have? */
 #ifdef __GNUC__
 #define GCC_VERSION (__GNUC__ * 100 + __GNUC_MINOR__)