Try harder to clean up on exit.

The fewer false positives are reported by tools like valgrind, the
easier it gets to spot actual leaks.

Change-Id: Ic7c73d48715d0899ad910088d44825988fa5b34a
Reviewed-by: Joerg Bornemann <joerg.bornemann@digia.com>
Improve the documentation on building Qbs.

Now lists the recognized qmake CONFIG options.

Change-Id: I183f97624566870e6b4d59d0c9e7f4fcb266e54f
Reviewed-by: Leena Miettinen <riitta-leena.miettinen@digia.com>
Reviewed-by: Christian Kandeler <christian.kandeler@digia.com>