Promote likely()/unlikely() to public header.

I have been using this in the relay to optimize some branches in inner loop
code, and I want to do that more broadly.  This is an idiom from the linux
kernel.

src/steamnetworkingsockets/steamnetworkingsockets_internal.h

@@ -107,6 +107,24 @@ struct iovec
 struct iovec;
 #endif
 
+// likely() and unlikely().  Branch hints
+// This is an idiom from the linux kernel
+#ifdef __GNUC__
+	#ifndef likely
+		#define likely(x) __builtin_expect (!!(x), 1)
+	#endif
+	#ifndef unlikely
+		#define unlikely(x) __builtin_expect (!!(x), 0)
+	#endif
+#else
+	#ifndef likely
+		#define likely(x) (x)
+	#endif
+	#ifndef unlikely
+		#define unlikely(x) (x)
+	#endif
+#endif
+
 // Internal stuff goes in a private namespace
 namespace SteamNetworkingSocketsLib {