New features
------------

CMake's `AUTOMOC` and `AUTOUIC` now cache information extracted when parsing
source files in `CMakeFiles/<ORIGIN>_autogen.dir/ParseCache.txt`.
This leads to faster `<ORIGIN>_autogen` target rebuilds, because source files
will be parsed again only if they're newer than the `ParseCache.txt` file.
The parse cache will be recomputed if it is older than the CMake executable.

`AUTOMOC` and `AUTOUIC` now check if `moc` or `uic` output files are older
than the `moc` or `uic` executable.  If an output file is older than the
compiler, it will be regenerated.  Therefore if a new `moc` or `uic` version
is installed, all output files will be regenerated.

`AUTOMOC` and `AUTOUIC` error and warning messages are more detailed.

Internal changes
----------------

`moc` and `uic` output file names are not computed in the `_autogen`
target anymore but in `cmQtAutoGenInitializer`.  This makes the available at
the configuration stage for improved dependency computations (to be done).

In `AutogenInfo.cmake`, equally sized lists for "source file names",
"source file flags" and "compiler output file names" are passed to the
`_autogen` target.  This replaces the separate file lists for
`AUTOMOC` and `AUTOUIC`.

Files times are read from the file system only once by using `cmFileTime`
instances instead of `cmQtAutoGenerator::FileSystem::FileIsOlderThan` calls.

All calls to not thread safe file system functions are moved to non concurrent
fence jobs (see `cmWorkerPool::JobT::IsFence()`).  This renders the
`cmQtAutoGenerator::FileSystem` wrapper class obsolete and it is removed.

Instead of composing a single large settings string that is fed to the
`cmCryptoHash`, now all setting sub strings are fed one by one to the
`cmCryptoHash` and the finalized result is stored.

The `std::mutex` in `cmQtAutoGenerator::Logger` is tagged `mutable` and most
`cmQtAutoGenerator::Logger` methods become `const`.

Outlook
-------

This patch provides the framework required to

- extract dependencies from `.ui` files in `AUTOUIC`.
  These will help to address issue
  #15420 "AUTOUIC: Track uic external inputs".

- generate adaptive `make` and `ninja` files in the `_autogen` target.
  These will help to address issue
  #16776 "AUTOUIC: Ninja needs two passes to correctly build Qt project".

- generate (possibly empty) `moc` and `uic` files for all headers instead of a
  `mocs_compilation.cpp` file.
  This will help to address issue
  #17277 "AUTOMOC: Provide a option to allow AUTOMOC to compile individual "
         "moc_x.cxx instead of including all in mocs_compilation.cxx"