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.
Didn't work. Tried that, and ``` --with-sdl-exec-prefix=#{prefix} ``` and ``` --with-sdl-prefix=#{HOMEBREW_PREFIX} --with-sdl-exec-prefix=#{prefix} ``` in combination, and then I tried disabling the compile-time test, only for it to refuse to link to the libs, and then it kept refusing to link to the libs even with a flag in place.