Fix Trojita's idea on where the plugins are going to be located

The old code mistakenly assumed that CMAKE_INSTALL_LIBDIR would alweays be
relative. That is not the case; see for example [1] [2] [3].

[1] http://lists.freedesktop.org/archives/packagekit/2013-September/026161.html
[2] http://www.cmake.org/pipermail/cmake/2012-February/049206.html
[3] https://bugzilla.redhat.com/show_bug.cgi?id=795542