[CMake] FindQt4.cmake allows programs to ignore CMAKE_INSTALL_PREFIX for plugins

[Andreas Pakulat]

Hi,

Currently FindQt4.cmake provides the path where the Qt4 installation has
put its own plugins, apparently to fulfil a bugreport that was filed
where somebody wanted to install a plugin. 

Now this directory completely circuumvents CMAKE_INSTALL_PREFIX and
makes it impossible to install a Qt4-based application that uses
QT4_PLUGIN_DIR to install plugins if the Qt4 is a system-version. This
should be prevented as it can break packaging systems or create
hard-to-track problems for the users.

Its not needed to install new Qt4 plugins into the qt4-prefix/plugins
dir, instead you can install them into any location and set
QT_PLUGIN_PATH to include that location. This is documented in the Qt4
api docs and a perfectly correct solution.

This is currently a problem for quite many KDE svn users that want to
install QCA (Qt Crypto API), thus I'm cc'ing Brad Hards.

I think the QT_PLUGINS_DIR variable should be removed again from
FindQt4.cmake, or at least it should have a big fat warning that users
might end up with installing into two locations, where they don't have
write access to one of them.

Andreas

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