Set a global property in the find_package implementation. Track and
reset that property in the find_dependency macro. Read the property
in FeatureSummary when determining whether to print output.
This means that packages which are found only as dependencies are not
listed by FeatureSummary, but if a project uses find_package elsewhere
directly, then it will be listed by FeatureSummary.
Suggested-by: Alex Merry
http://thread.gmane.org/gmane.comp.kde.devel.frameworks/10640