Add Ada syntax checker with GNAT

Based on #414, but with some changes

- Use `gnatmake` directly instead of `gnat compile`
- Move generated files to a temporary directory with `-D`
- Hardcode `-gnatef` and `-gnatf` options. Imho these shouldn't be in `flycheck-gnat-warnings`
- Remove `wu` from `flycheck-gnat-warnings`, since it's included in `gnatwa` anyway.

@nordlow Comments?