Refactor the code in `tools/osx/builder` somewhat and make it cleaner.

Move a few table processing loops into functions to make things less
ugly.

Delete dists not listed in the table (e.g. when they are updated) and
when a dist is re-downloaded, delete the tree and the target file to
force a rebuild.

Use number version for gettext instead of `-latest`.

Specify full dist target, e.g. `lib/libfoo.a` instead of just
`libfoo.a`.