[error] disable error checking enforcement for old versions of clang (#4932)
[error] disable error checking enforcement for old versions of clang (#4932)

With clang older than 3.9, the warn_unused_result feature works only
with functions and classes, not enums. This disables it if the clang
version is too old.
openthread/openthread,librasungirl/openthread,openthread/openthread,openthread/openthread,librasungirl/openthread,abtink/openthread,chshu/openthread,jwhui/openthread,lanyuwen/openthread,jwhui/openthread,bukepo/openthread,bukepo/openthread,bukepo/openthread,abtink/openthread,chshu/openthread,lanyuwen/openthread,openthread/openthread,lanyuwen/openthread,chshu/openthread,srickardti/openthread,lanyuwen/openthread,srickardti/openthread,bukepo/openthread,srickardti/openthread,chshu/openthread,abtink/openthread,lanyuwen/openthread,srickardti/openthread,jwhui/openthread,jwhui/openthread,librasungirl/openthread,abtink/openthread,librasungirl/openthread,chshu/openthread,lanyuwen/openthread,chshu/openthread