Added SDL_LinuxSetThreadPriority() to directly set the priority of a Linux thread (tid)

This function tries using RealtimeKit connecting over DBUS as needed.

include/SDL_system.h

@@ -76,6 +76,18 @@ extern DECLSPEC SDL_bool SDLCALL SDL_DXGIGetOutputInfo( int displayIndex, int *a
 #endif /* __WIN32__ */
 
 
+/* Platform specific functions for Linux */
+#ifdef __LINUX__
+
+/**
+   \brief Sets the UNIX nice value for a thread, using setpriority() if possible, and RealtimeKit if available.
+
+   \return 0 on success, or -1 on error.
+ */
+extern DECLSPEC int SDLCALL SDL_LinuxSetThreadPriority(Sint64 threadID, int priority);
+ 
+#endif /* __LINUX__ */
+	
 /* Platform specific functions for iOS */
 #if defined(__IPHONEOS__) && __IPHONEOS__