Fix documentation build with qbs.

Apparently, the qdoc in Qt 5.1 no longer allows a relative path as
the source directory.

Change-Id: Ia4de7a070020ae1951affa5653dbf73475d1fed3
Reviewed-by: Joerg Bornemann <joerg.bornemann@digia.com>
Log command-specific environment variables when tracing.

In addition to the command line.

Change-Id: I0ef3576b4e3eb810fd923bef23799a7fff62aae6
Reviewed-by: Joerg Bornemann <joerg.bornemann@digia.com>