Only search for Boost if there isn't already an include path present.
Only search for Boost if there isn't already an include path present.

This allows a parent project to setup boost themselves before adding
this library if they want to customize the settings.

sirikata/json-spirit,upsoft/json-spirit,infinit/json-spirit,infinit/json-spirit,mickem/json-spirit,upsoft/json-spirit,andmaj/json-spirit,sirikata/json-spirit,andmaj/json-spirit,mickem/json-spirit