Add support for the Apache license

[iustin]

This patch adds the Apache license (http://www.apache.org/licenses/)
as a versioned license. I'm not entirely sure this is correct, since
they do fractional versions (1.1, 2.0) as opposed to the FSF ones
which are integral (2, 3, etc.); so the fact that the version is an
integer doesn't map well.

I've also updated slightly the license parse test, but I'm not sure
that the instructions for running the tests are correct (I don't see
the "license parsers" test being shown when running
./test/dist/build/suite/suite).

[iustin]

Uh, I fear the old discussion has been "eaten" by my force push on this branch/pull request…

[kosmikus]

I think I'd still go for [2] and not [2,0]. GPL also has version 3.0, and we represent it using [3]. I just wanted to make clear that version 1.1 could without trouble be represented as [1,1].

[iustin]

Sure, I'm not particular to either [2] or [2, 0]. Shall I re-send as [2]?

I can also add one patch on top changing it, whatever works best.

[iustin]

I've done some investigation on this. The GPL 3 license says, under "How to Apply These Terms to Your New Programs", the following:

This program is free software: you can redistribute it and/or modify
it under the terms of the GNU General Public License as published by
the Free Software Foundation, either version 3 of the License, or
(at your option) any later version.

Note the unqualified 3.

On the other hand, Apache says under "APPENDIX: How to apply the Apache License to your work":

Licensed under the Apache License, Version 2.0 (the "License");
you may not use this file except in compliance with the License.
You may obtain a copy of the License at …

Note the qualified 2.0, and thus my original intention of marking this explicitly as such.

Anyway, I have a patch ready changing this to a plain 2, just let me know which one you prefer.

[kosmikus]

You're making a good point. I'm willing to go with [2,0] then.

[kosmikus]

Your cabal init changes look fine to me. I'm still slightly worried about backwards compatibility issues. @dcoutts ?

[iustin]

Small ping?

If you can explain the backwards compat issues (I didn't understand them), I can look into it. Thanks!

[kosmikus]

I've talked to @dcoutts, and I think the agreement was that there are no backwards compat issues. I'll merge.

[dcoutts]

Yep, I'm happy, code looks fine.

Only question is if we want to allow specifying an unversioned Apache license? We allow unversioned for GPL and LGPL only as a backwards compat thing, because we didn't initially have versioned licenses at all.

Actually, looking at the code I think we could get away with dropping the unversioned GPL and LGPL from the knownLicenses. All it means is you'll get a build warning (and such packages will not be able to be uploaded to hackage).