useorocos: set ${PACKAGE}_FOUND variable to true if the package has been found in orocos_find_package()

This follows cmake standards for similar commands like find_package().
Also added a FOUND check to orocos_use_package() although it's not really required here.
Merge pull request #9 from jhu-lcsr-forks/set_found_variable

Added ${PACKAGE}_FOUND variable output in orocos_find_package()