configure.ac: don't fail in x26410b if library not found by default

Only fail if user explicitly enabled x26410b option and suitable libraries
are not found.

configure.ac

@@ -2813,7 +2813,9 @@ AC_ARG_WITH(x26410b-tree,
           VLC_ADD_CFLAGS([x26410b], [-DPTW32_STATIC_LIB])
         fi
     ], [
+       if test "${enable_x26410b}" == "yes"; then
         AC_MSG_ERROR([x26410b module doesn't work without staticly compiled libx264.a])
+       fi
     ])
   fi
 fi