Calling `std::string::front()` on an empty string results
in an undefined behavior by the C++ standard.
In gcc8 it causes an assertion to fail.
This adds a check to `AUTOGEN` if a file to read is empty
and in case avoids the use of an empty `std::string` buffer.
Closes#17793
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.
The AUTOMOC/AUTOUIC generator and the AUTORCC generator are
refactored to use a libuv loop internally.
Closes#17422.
In the AutogenInfo.cmake file the separator for nested lists
was `@LSEP@` which led to a speed regression because the `@`
character triggered an (unsuccessful) expression evaluation.
By setting the policy version of the CMake instance in the
`_autogen` target to 3.9, the OLD `@` evaluating behavior
controlled by policy CMP0053 is disabled.
Also the nested lists separator string is changed to `<<<S>>>`,
which solves the problem twofold.
Closes#17570
