Fix api.pri.

Change-Id: I358a423e5a4e97f96b127fa94fbccaf664c77de6
Reviewed-by: Jake Petroules <jake.petroules@petroules.com>
Reviewed-by: Joerg Bornemann <joerg.bornemann@digia.com>
Fix a number of problems with the NSIS module.

- Add support for building installers outside of Windows.
- Add missing qbs. qualifiers
- Fix compressor handling

Task-number: QBS-414

Change-Id: I1df290d5dd79cb2fea9ab27feefdde21a46c69d7
Reviewed-by: Joerg Bornemann <joerg.bornemann@digia.com>