add missing os/2 apientry defs to SDL_opengl.h
diff --git a/include/SDL_opengl.h b/include/SDL_opengl.h
index f8d80c7..314dd57 100644
--- a/include/SDL_opengl.h
+++ b/include/SDL_opengl.h
@@ -97,6 +97,13 @@
#elif defined(__CYGWIN__) && defined(USE_OPENGL32) /* use native windows opengl32 */
# define GLAPI extern
# define GLAPIENTRY __stdcall
+#elif defined(__OS2__) || defined(__EMX__) /* native os/2 opengl */
+# define GLAPI extern
+# define GLAPIENTRY _System
+# define APIENTRY _System
+# if defined(__GNUC__) && !defined(_System)
+# define _System
+# endif
#elif (defined(__GNUC__) && __GNUC__ >= 4) || (defined(__SUNPRO_C) && (__SUNPRO_C >= 0x590))
# define GLAPI __attribute__((visibility("default")))
# define GLAPIENTRY