build: example binaries use TARGETS again to handle rpath correctly

I'm sorry, after #76 it fails to remove built rpath from example binaries (onion-fileserver, onion-cairo-example). Cmake only removes it when TARGETS instead of PROGRAMS is used.

The proper way to rename targets is just to add
set_target_properties(oldname PROPERTIES OUTPUT_NAME newname)

The install lines are now again the same like before we started all our
renaming trials.