Allow found-variable to be named as package

Boost does not set the BOOST_FOUND variable to indicate that the
package is found (as it would if it had used the usual routine
find_package_handle_standard_args), but rather Boost_FOUND. This
patch tests for that condition, and print some more helpful debug
messages if a prerequisite is missing.
Merge pull request #323 from rolk/323_case

Allow found-variable to be named as package