Allow building for Qt5 even if Qt6 was detected.

[pasbi]

My system provides both Qt5 and Qt6.
Usually I use Qt6, but some projects still require Qt5 (some of which require libQGLViewer, too).

So it seems I wasn't able to build libQGLViewer for Qt5 unless

• I uninstalled Qt6 from my system (not an option)
• or made changes to this part of the sources (not nice, I prefer having no local changes for 3rd party source in my environment).

# This will find the Qt files.
find_package(Qt6 COMPONENTS Core Widgets Xml OpenGL OpenGLWidgets)
if (Qt6_FOUND)
    message("Building with Qt6")
    set(QtLibs Qt::Core Qt::Widgets Qt::Xml Qt::OpenGL Qt::OpenGLWidgets)
else()
    find_package(Qt5 5.15 REQUIRED COMPONENTS Core Widgets Xml OpenGL)
    message("Building with Qt5")
    set(QtLibs Qt::Core Qt::Widgets Qt::Xml Qt::OpenGL)
endif()

I guess the intention behind this logic is to "automatically" pick the "right" Qt version without bothering the user.
I suppose libQGLviewer built with Qt6 doesn't work in applications build with Qt5 (if that assumption is wrong, please let me know and close the issue).
Hence, I think it's required to let the user decide explicitly because only the user knows what they're building libQGLViewer for.

One could use -DCMAKE_DISABLE_FIND_PACKAGE_Qt6=On, but that feels more like a hacky workaround and not properly documented.

Feature request: add an option to explicitly chose the Qt to be used for building libQGLViewer.