Add cmake option WANT_PERL_DOCS to disable doc generation.

[ctrlaltca]

Also:

• ensure Qt version is reported correctly by cmake.
• KviApplication::checkGlobalKvircDirectory: don't test for help subdir (it may not exist if WANT_PERL_DOCS=off)
• Cmake: use the imported target for Threads instead of CMAKE_THREAD_LIBS_INIT
  Fix Small optimization of build process: QT_VERSION_MAJOR and WANT_PERL_DOCS #2634

[VVD]

Your patch for PERL options (4d6af2e) is better than mine - can build with perl scripting without documentation, but with my patch it's impossible.
But your patch for Threads (2b4af7b) doesn't work - same link error.

[ctrlaltca]

But your patch for Threads (2b4af7b) doesn't work - same link error.

Can you please test adding 03fb5fb ?

[VVD]

But your patch for Threads (2b4af7b) doesn't work - same link error.

Can you please test adding 03fb5fb ?

With both patches build without errors.