cmake: fix finding Qt private headers with Qt >=6.10

[whitslack]

Qt private header directories only get added to the include search path when the Qt6::${component}Private library is added to the target.

[Tatsh]

This gets past the configure stage and gets 99% into the build but fails for me:

webkit-1ab4d21d082e89fe1837c0279467a6da18c78cf0/Tools/QtTestBrowser/launcherwindow.h:47:10: fatal error: private/qeventpoint_p.h: No such file or directory
   47 | #include <private/qeventpoint_p.h>
      |          ^~~~~~~~~~~~~~~~~~~~~~~~~

This file does exist for me at /usr/include/qt6/QtGui/6.10.1/QtGui/private/qeventpoint_p.h but I am assuming the include path is not being set properly.

The command has these -isystem flags:

-isystem /usr/include/qt6/QtGui \
-isystem /usr/include/qt6 \
-isystem /usr/include/qt6/QtCore \
-isystem /usr/include/qt6/QtDBus \
-isystem /usr/include/qt6/QtNetwork \
-isystem /usr/include/qt6/QtWidgets \
-isystem /usr/lib64/qt6/mkspecs/linux-g++ \
-isystem /usr/include/qt6/QtPrintSupport \

All of the -I flags are only referencing directories in the source.

-isystem /usr/include/qt6/QtGui/6.10.1/QtGui \
-isystem /usr/include/qt6/QtGui/6.10.1 \
-isystem /usr/include/qt6/QtCore/6.10.1 \

If I run this manually with the above added, it compiles that file.

[Tatsh]

Please consider adding https://github.com/Tatsh/tatsh-overlay/blob/master/dev-qt/qtwebkit/files/qtwebkit-fix-qttestbrowser-build.patch to this PR.

[whitslack]

@Tatsh: Thank you for testing. I wasn't building with tests enabled, so I didn't catch the missing piece. Do you know for certain that NetworkPrivateCorePrivate is required here? Your error message only mentioned a private header from GuiPrivate.

[Tatsh]

I assume you mean CorePrivate. Once I ran with the GuiPrivate includes another error occurred regarding CorePrivate.

[whitslack]

@Tatsh: Yes, I did. And does it not work if you append to QtTestBrowser_PRIVATE_LIBRARIES (rather than QtTestBrowser_LIBRARIES)? (The former will not be propagated to dependent targets.)