Fix SDL detection on old distros (no pkgconfig file)

Thanks to Francois Andriot for the patch!

ConfigureChecks.cmake

@@ -106,7 +106,12 @@ if( WITH_LIBVISUAL )
 
   pkg_search_module( SDL sdl )
   if( NOT SDL_FOUND )
-    tde_message_fatal( "SDL are required, but not found on your system" )
+    check_include_file( SDL/SDL.h HAVE_SDL_H )
+    if( NOT HAVE_SDL_H )
+      tde_message_fatal( "SDL are required, but not found on your system" )
+    endif ( )
+    set ( SDL_INCLUDE_DIRS /usr/include/SDL )
+    set ( SDL_LIBRARIES SDL pthread )
   endif( )
 
   pkg_search_module( LIBVISUAL libvisual-0.4 )