Run license checker in qmake

Check for a valid license not only in configure, but also in qmake.
To limit the runtime overhead we cache the day of the last run in
a .stash file. This allows us to run licheck only for the top-level
qmake call, and only once per day.

This requires an updated licheck executable that supports the new
check mode.

[ChangeLog][Tools][qmake] For commercial builds, qmake now checks for
a valid Qt license. This requires setting up a Qt Account (or
.qt-license file) on the development machine.

Change-Id: I2c2a05a4602cc661560568b76ddf520cb8134769
Reviewed-by: Oswald Buddenhagen <oswald.buddenhagen@theqtcompany.com>