FeatureSummary: Don't list transitive package dependencies

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
Merge topic 'FeatureSummary-no-transitive' into next

a6971f65 FeatureSummary: Don't list transitive package dependencies