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.

(This should really have been part of #322, but didn't surface until the dependant modules, for some reason)