Build issue fix: QT_MKSPECS_DIR is not defined in Qt5 cmake files, so it must be
defined manually.

Solution inspired by http://mail.kde.org/pipermail/kde-frameworks-devel/2012-October/000990.html
Updated pkgconfig file to require Qt5
Print more warnings when plugin loading fails
Fixed a bug at plugin loading.
Nodes with empty dependency list weren't being added to dependency tree
fix wrong spelling in include file