[LyX/master] Allow compiling with Qt6

[Yu Jin]

Just seen this one and wanted to try it out obviously :)
But it doesn't work for me on Windows. Using CMake GUI I notice that the
dropdown menu LYX_USE_QT does not contain QT6, just QT5, QT4 and AUTO as
before. When I choose AUTO it fails with:

CMake Error at development/cmake/modules/FindQt4.cmake:568 (message):
  Could NOT find QtCore.  Check
  C:/lyx/masterbuild64Qt6/CMakeFiles/CMakeError.log for more details.
Call Stack (most recent call first):
  CMakeLists.txt:804 (find_package)

It does find some QT6 stuff correctly though, like QT_PLUGINS_DIR, but not
everything, like QtCore.
-- 
    Eugene
-------------- next part --------------
An HTML attachment was scrubbed...
URL: <http://lists.lyx.org/pipermail/lyx-devel/attachments/20210316/869b3437/attachment.html>