[LyX/master] Enable QWindowsMimeMetafile with Qt6

[Enrico Forestieri]

On Sun, Oct 17, 2021 at 04:16:22PM +0200, Kornel Benko wrote:
> Am Sun, 17 Oct 2021 15:29:09 +0200
> schrieb Enrico Forestieri <forenr at lyx.org>:
> 
> > On Sun, Oct 17, 2021 at 02:48:31PM +0200, Enrico Forestieri wrote:
> > > commit 771f30e9463d8d7bf7acd4dfc564f462b28eae16
> > > Author: Enrico Forestieri <forenr at lyx.org>
> > > Date:   Sun Oct 17 15:06:05 2021 +0200
> > > 
> > >     Enable QWindowsMimeMetafile with Qt6
> > >     
> > >     The QWinMime class has been removed in Qt6 but the functionality
> > >     is still present. However, one has to allow inclusion of private
> > >     headers and register the mime handling to the QWindowsApplication
> > >     native interface.  
> > 
> > Kornel, according to
> > https://www.qt.io/blog/qt-extras-modules-in-qt-6#qwinmime
> > to enable private headers with cmake one has to add a project dependency
> > to Qt::GuiPrivate. I honestly don't know how to do that. This only
> > matters for Windows and after this commit compilation cannot succeed
> > when configuring with cmake if private headers are not enabled.
> > 
> 
> My only idea is to use
> 	if (MINGW AND LYX_USE_QT MATCHES "QT6")
> 		add_dependencies(${_lyx} Qt::CorePrivate Qt::GuiPrivate)
> 	endif()
> somewhere at src/CMakeLists.txt:125

That did not work. However, I found a solution here:
https://stackoverflow.com/questions/65060435/add-qt-private-headers-to-cmake-project
and committed the corresponding patch at e8b366d4.

-- 
Enrico