COMMON: Add a PHAETHON_FALLTHROUGH macro

While gcc can parse the "// Fallthrough" comments we use to mark
switch case fallthroughs, clang can not. Instead, clang wants a
[[clang::fallthrough]] attribute.

So we use the PHAETHON_FALLTHROUGH macro to select the appropriate
way to mark a fallthrough.

src/common/system.h

@@ -179,6 +179,41 @@
 	#define IGNORE_UNUSED_VARIABLES _Pragma("GCC diagnostic ignored \"-Wunused-variable\"")
 #endif
 
+/** PHAETHON_FALLTHROUGH is an annotation to suppress compiler warnings about switch
+ *  cases that fall through without a break or return statement. PHAETHON_FALLTHROUGH
+ *  is only needed on cases that have code.
+ *
+ *  Based on Mozilla's MOZ_FALLTHROUGH and Boost's BOOST_FALLTHROUGH.
+ *
+ *  switch (foo) {
+ *    case 1: // These cases have no code. No fallthrough annotations are needed.
+ *    case 2:
+ *    case 3: // This case has code, so a fallthrough annotation is needed!
+ *      foo++;
+ *      PHAETHON_FALLTHROUGH;
+ *    case 4:
+ *      return foo;
+ *  }
+ */
+#ifndef __has_cpp_attribute
+	#define __has_cpp_attribute(x) 0
+#endif
+
+#if __has_cpp_attribute(clang::fallthrough)
+	#define PHAETHON_FALLTHROUGH [[clang::fallthrough]]
+#elif __has_cpp_attribute(gnu::fallthrough)
+	#define PHAETHON_FALLTHROUGH [[gnu::fallthrough]]
+#elif defined(_MSC_VER)
+	/*
+	 * MSVC's __fallthrough annotations are checked by /analyze (Code Analysis):
+	 * https://msdn.microsoft.com/en-us/library/ms235402%28VS.80%29.aspx
+	 */
+	#include <sal.h>
+	#define PHAETHON_FALLTHROUGH __fallthrough
+#else
+	#define PHAETHON_FALLTHROUGH // Fallthrough
+#endif
+
 //
 // Fallbacks for various functions
 //