sdl: fixing dom is a moron

Deals with the issue flagged up in [this commit](https://github.com/Homebrew/homebrew/commit/0d6d0213dc4cc406cdfdcb5ba742b361a0ab2210). Should have caught this at the time. I am an idiot.
How about passing `--with-sdl-prefix=#{prefix}` to `configure` instead?