Indicate system includes for relevant SDL headers

acinclude.m4

@@ -55,7 +55,7 @@ dnl
 #include <stdio.h>
 #include <stdlib.h>
 #include <string.h>
-#include "SDL.h"
+#include <SDL.h>
 
 char*
 my_strdup (char *str)
@@ -132,7 +132,7 @@ int main (int argc, char *argv[])
           LIBS="$LIBS $SDL_LIBS"
           AC_TRY_LINK([
 #include <stdio.h>
-#include "SDL.h"
+#include <SDL.h>
 ],      [ return 0; ],
         [ echo "*** The test program compiled, but did not run. This usually means"
           echo "*** that the run-time linker is not finding SDL or finding the wrong"