Disable SDL if not used

Patch by Henrik Dohlmann

http://bugs.gentoo.org/113441

--- configure.in
+++ configure.in
@@ -587,9 +587,14 @@
 dnl
 
 dnl Check for SDL
-AM_PATH_SDL(1.2.5,,)
-HSDL="$SDL_CFLAGS"
-LIBSDL="$SDL_LIBS"
+if test "$use_sdl" = "yes" ; then
+  AM_PATH_SDL(1.2.5,,)
+  HSDL="$SDL_CFLAGS"
+  LIBSDL="$SDL_LIBS"
+else
+  HSDL=""
+  LIBSDL=""
+fi
 AC_SUBST(LIBSDL)
 AC_SUBST(HSDL)