Conditionally build with MPL and BSD2 support.

@nomeata This is the error I was receiving.
```
$ cabal build
Building cabal-debian-4.19...
Preprocessing library cabal-debian-4.19...
[11 of 28] Compiling Debian.Policy    ( src/Debian/Policy.hs, dist/build/Debian/Policy.o )

src/Debian/Policy.hs:362:7:
    Not in scope: data constructor ‘Cabal.BSD2’
    Perhaps you meant one of these:
      ‘Cabal.BSD3’ (imported from Distribution.License),
      ‘Cabal.BSD4’ (imported from Distribution.License)

src/Debian/Policy.hs:366:7:
    Not in scope: data constructor ‘Cabal.MPL’
    Perhaps you meant one of these:
      ‘Cabal.GPL’ (imported from Distribution.License),
      ‘Cabal.AGPL’ (imported from Distribution.License),
      ‘Cabal.LGPL’ (imported from Distribution.License)

src/Debian/Policy.hs:380:23:
    Not in scope: data constructor ‘Cabal.BSD2’
    Perhaps you meant one of these:
      ‘Cabal.BSD3’ (imported from Distribution.License),
      ‘Cabal.BSD4’ (imported from Distribution.License)
```

MPL and BSD2 support was added in Cabal 1.19.2, Here are some commits:

https://github.com/haskell/cabal/commit/1e534
https://github.com/haskell/cabal/commit/cdc24
https://github.com/haskell/cabal/commit/0a65decc

Hope this helps, I'll be happy to fix any other issues here.