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

Patch contributed by Sylvain

include/SDL_test_font.h

@@ -27,8 +27,8 @@
  *  This code is a part of the SDL2_test library, not the main SDL library.
  */
 
-#ifndef _SDL_test_font_h
-#define _SDL_test_font_h
+#ifndef SDL_test_font_h_
+#define SDL_test_font_h_
 
 #include "begin_code.h"
 /* Set up for C function definitions, even when using C++ */
@@ -71,6 +71,6 @@ int SDLTest_DrawString( SDL_Renderer * renderer, int x, int y, const char *s );
 #endif
 #include "close_code.h"
 
-#endif /* _SDL_test_font_h */
+#endif /* SDL_test_font_h_ */
 
 /* vi: set ts=4 sw=4 expandtab: */