fix: rename Boost_INCLUDE_DIR to Boost_INCLUDE_DIRS

cmake sets Boost_INCLUDE_DIRS but not Boost_INCLUDE_DIR.

This fixes build issues where cmake complains that Boost_INCLUDE_DIR is set to NOTFOUND

This obsoletes !37 (closed)

based on comment by @nalvarez

The documentation of the module says you should use DIRS, and DIR is just an internal variable to cache the search result between cmake runs. So yes, you should replace Boost_INCLUDE_DIR with Boost_INCLUDE_DIRS everywhere.