This introduces concurrent thread processing in the _autogen
target wich processes AUTOMOC and AUTOUIC.
Source file parsing is distributed among the threads by
using a job queue from which the threads pull new parse jobs.
Each thread might start an independent moc or uic process.
Altogether this roughly speeds up the AUTOMOC and AUTOUIC build
process by the number of physical CPUs on the host system.
The exact number of threads to start in the _autogen target
is controlled by the new AUTOGEN_PARALLEL target property which
is initialized by the new CMAKE_AUTOGEN_PARALLEL variable.
If AUTOGEN_PARALLEL is empty or unset (which is the default)
the thread count is set to the number of physical CPUs on
the host system.