[Qt WebEngine] More configure-time checks and documentation for required libraries

CMake Error at /home/USER/qt-everywhere-src-6.5.3/qtwebengine/cmake/Gn.cmake:75 (message):
  

  -- GN FAILED

  ERROR at //build/config/linux/pkg_config.gni:104:17: Script returned
  non-zero exit code.

      pkgresult = exec_script(pkg_config_script, args, "value")
                  ^----------

  Current dir: /home/USER/build-qtwebengine/src/core/RelWithDebInfo/x86_64/

  Command: /usr/bin/python3
  /home/USER/qt-everywhere-src-6.5.3/qtwebengine/src/3rdparty/chromium/build/config/linux/pkg-config.py
  -p /usr/bin/pkg-config xkbcommon xkbfile

  Returned 1.

  stderr:

  

  Package xkbfile was not found in the pkg-config search path.

  Perhaps you should add the directory containing `xkbfile.pc'

  to the PKG_CONFIG_PATH environment variable

  No package 'xkbfile' found

  Could not run pkg-config.

  

  See
  /home/USER/qt-everywhere-src-6.5.3/qtwebengine/src/core/ozone/BUILD.gn:8:3:
  whence it was called.

    pkg_config("xkb") {
    ^------------------

  See /home/USER/build-qtwebengine/src/core/RelWithDebInfo/x86_64/BUILD.gn:611:15: which
  caused the file to be included.

      deps += [ "$_ozone_extra_directory:qt" ]
                ^---------------------------

CMake Error at cmake/Gn.cmake:75 (message):
  

  -- GN FAILED

  ERROR at //printing/BUILD.gn:406:14: Script returned non-zero exit code.

        libs = exec_script("cups_config_helper.py",
               ^----------

  Current dir:
  /home/USER/qt-everywhere-src-6.5.3/qtwebengine/src/pdf/Release/x86_64/


  Command: /usr/bin/python3
  /home/USER/qt-everywhere-src-6.5.3/qtwebengine/src/3rdparty/chromium/printing/cups_config_helper.py
  --libs-for-gn

  Returned 1.

  stderr:

  

  Traceback (most recent call last):

    File "/home/USER/qt-everywhere-src-6.5.3/qtwebengine/src/3rdparty/chromium/printing/cups_config_helper.py", line 108, in <module>
      sys.exit(main())
    File "/home/USER/qt-everywhere-src-6.5.3/qtwebengine/src/3rdparty/chromium/printing/cups_config_helper.py", line 92, in main
      flags = run_cups_config(cups_config, mode)
    File "/home/USER/qt-everywhere-src-6.5.3/qtwebengine/src/3rdparty/chromium/printing/cups_config_helper.py", line 35, in run_cups_config
      cups = subprocess.Popen([cups_config, '--cflags', '--ldflags', '--libs'],
    File "/usr/lib/python3.10/subprocess.py", line 971, in __init__
      self._execute_child(args, executable, preexec_fn, close_fds,
    File "/usr/lib/python3.10/subprocess.py", line 1863, in _execute_child
      raise child_exception_type(errno_num, err_msg, err_filename)

  FileNotFoundError: [Errno 2] No such file or directory: 'cups-config'

  

  See //BUILD.gn:274:15: which caused the file to be included.

      deps += [ "//printing:printing_unittests" ]
                ^------------------------------