Revert "configure.ac: error when unknown target"

This reverts commit acf6b8cdfc7176318ea2641bb1dc14a79d8f69a7.

configure.ac

@@ -698,8 +698,6 @@ case "$target" in
   starpu_darwin=yes
   AC_DEFINE(STARPU_HAVE_DARWIN, [1], [Define this on darwin.])
   ;;
-*)
-  AC_ERROR(target $target unknown)
 esac
 AM_CONDITIONAL([STARPU_HAVE_WINDOWS], [test "x$starpu_windows" = "xyes"])
 AM_CONDITIONAL([STARPU_LINUX_SYS], [test "x$starpu_linux" = "xyes"])