Provide cmake exports

sunderme requested to merge sunderme/poppler:cmakeExports into master

Cmake allows integrating libraries comfortably via the find_package mechanism. For this to work, libraries need to export targets which can be used by other projects. This pull-request presents the initial work which provides this interface for poppler,poppler-cpp and poppler-qt6. Missing is a config-version.cmake file and possibly a better solution for components. Other interfaces may be added later.

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