qt4 vs qt5 build for dependencies

[Max Brazhnikov]

On Fri, 05 Apr 2013 23:22:09 +1300 Ben Cooksley wrote:
> On Fri, Apr 5, 2013 at 10:27 PM, David Faure <faure+bluesystems at kde.org> wrote:
> > When I ported attica, soprano, libdbusmenu-qt and other such "kdesupport-
> > level" dependencies to Qt5, I used
> >
> > find_package(Qt5Core QUIET)
> > if (Qt5Core_FOUND)
> >   message(STATUS "Building with Qt5 support")
> >   ...
> > else()
> >   find_package(Qt4 REQUIRED)
> > endif()
> >
> > i.e. use Qt5 if it's in the user's environment (CMAKE_PREFIX_PATH) otherwise
> > look for Qt4.
> > I thought this was perfect, it made things "work out of the box" without any
> > manual intervention, for both types of users.
> > And it didn't require a separate branch for the qt5 support (we only have this
> > in a few modules which started to use qt5 stuff, but for for the majority of
> > modules, there's no active development, so branching is not wanted).
> >
> > Now here's the problem: nowadays distros are starting to have Qt5 in /usr,
> > i.e. it's being found automatically. Which makes it impossible to build this
> > code for Qt4.
> >
> > So I'm afraid I have to drop the idea of automatic detection, and let people
> > choose explicitely. My current idea would be to reintroduce -DQT5_BUILD=true
> > (like we had in kdelibs at some point; but this discussion isn't about kdelibs
> > obviously), i.e. default to the Qt4 build, since that's what KDE SC releases
> > are still about.
> >
> > Later on, when we're about to release KF5, we want to turn this around, i.e.
> > make it easy to build things for Qt5, and if we still need qt4 support at that
> > point, adding a -DQT4_BUILD=true (and dropping the QT5_BUILD variable,
> > obviously).
> >
> > Opinions? Better ideas?
> 
> Assuming the user had given no guidance, would it be possible to
> attempt a fallback to Qt 5 if it was not possible to find Qt 4?
> Obviously if the user gives guidance that should override.
> 
> I'm otherwise neutral - just let me know beforehand please so I can
> adjust the CI system to pass the new argument(s).

Please, allow users to choose Qt version to build with. Automatic detection is a pain
for source based distributions.

Thanks,
Max