BUILTIN and --with-*-tree cleanup in configure.ac

Now that BUILTINS are deprecated, we still need to do some cleanup. This can be witnessed by the fact that we had bug #1682 (closed)

As some might understand i'm a bit hesitant to create fixes here, especially for things relating to win32, beos and xvideo plugins. I'm also wondering if any of those --with-*-tree options is still working AT ALL atm. I'm just not sure.