qt5.6: darwin compatibility

- qtbase builds on darwin
- Downstream qmake and cmake build configurations supported

Darwin-specific changes:

Prevent `qmake -project` from adding build support files in the temp
directory to the project.

Shrink the temp directory used for building with qmake.

The issue here is that `lndir` recreates the entire directory structure
of the module we are merging into the temporary build environment. On
darwin, the framework directory structures mean that this shadow
directory tree is 17MB even though the file contents are all symlinks
into the store.

To address this, we symlink the framework directories explicitly and
use `lndir` for everything else.