config: Make sure HAVE_TRUNC and HAVE_TRUNCF are defined as appropriate.

The configure/cmake scripts were checking for these functions but we didn't
have the SDL_config.h.* pieces in place. The other config headers are best
guesses.

include/SDL_config_os2.h

@@ -180,5 +180,7 @@
 /* #undef HAVE_SQRTF */
 #define HAVE_TAN 1
 /* #undef HAVE_TANF */
+#define HAVE_TRUNC 1
+/* #undef HAVE_TRUNCF */
 
 #endif /* SDL_config_os2_h_ */