Renaming of guard header names to quiet -Wreserved-id-macro

Patch contributed by Sylvain

src/thread/SDL_systhread.h

@@ -22,8 +22,8 @@
 
 /* These are functions that need to be implemented by a port of SDL */
 
-#ifndef _SDL_systhread_h
-#define _SDL_systhread_h
+#ifndef SDL_systhread_h_
+#define SDL_systhread_h_
 
 #include "SDL_thread.h"
 #include "SDL_thread_c.h"
@@ -65,6 +65,6 @@ extern SDL_Thread *
 SDL_CreateThreadInternal(int (SDLCALL * fn) (void *), const char *name,
                          const size_t stacksize, void *data);
 
-#endif /* _SDL_systhread_h */
+#endif /* SDL_systhread_h_ */
 
 /* vi: set ts=4 sw=4 expandtab: */

src/thread/SDL_thread_c.h

@@ -20,8 +20,8 @@
 */
 #include "../SDL_internal.h"
 
-#ifndef _SDL_thread_c_h
-#define _SDL_thread_c_h
+#ifndef SDL_thread_c_h_
+#define SDL_thread_c_h_
 
 #include "SDL_thread.h"
 
@@ -90,6 +90,6 @@ extern SDL_TLSData *SDL_Generic_GetTLSData(void);
  */
 extern int SDL_Generic_SetTLSData(SDL_TLSData *data);
 
-#endif /* _SDL_thread_c_h */
+#endif /* SDL_thread_c_h_ */
 
 /* vi: set ts=4 sw=4 expandtab: */

src/thread/pthread/SDL_sysmutex_c.h

@@ -20,13 +20,13 @@
 */
 #include "../../SDL_internal.h"
 
-#ifndef _SDL_mutex_c_h
-#define _SDL_mutex_c_h
+#ifndef SDL_mutex_c_h_
+#define SDL_mutex_c_h_
 
 struct SDL_mutex
 {
     pthread_mutex_t id;
 };
 
-#endif /* _SDL_mutex_c_h */
+#endif /* SDL_mutex_c_h_ */
 /* vi: set ts=4 sw=4 expandtab: */