only setting OHATTRIBUTEDLABEL_WARN_ABOUT_KNOWN_ISSUES and OHATTRIBUTEDLABEL_WARN_ABOUT_OLD_API if they where not defined otherwise before.
Merge pull request #143 from 343max/makeWarningsOptional

Only setting OHATTRIBUTEDLABEL_WARN_ABOUT_KNOWN_ISSUES and OHATTRIBUT… …
…EDLABEL_WARN_ABOUT_OLD_API if they where not defined otherwise before.