Put a system-wide lock on the build directory.

So that two qbs instances can't overwrite each other's
build graphs. In practice, this is probably most relevant
for IDEs, as these hold a build graph for potentially long
periods of time.
Facilitates QLockFile that was introduced in Qt 5.1. For older
Qt versions, no locking happens.

Task-number: QBS-162
Change-Id: Ib598617fb1742eb57b6a017f40b9631d1d54e627
Reviewed-by: Joerg Bornemann <joerg.bornemann@digia.com>