Fix warn_unused_result for clang < 4
Fix OSX Travis CI runner. Clang < 4 does not implement warn_unused_result though the advertised gcc version implies it.
Checklist
-
Commits have Signed-off-by:
with name/author being identical to the commit author -
Code modified for feature -
CI timeout is 2h or higher (see Settings/CICD/General pipelines/Timeout)
Reviewer's checklist:
-
Any issues marked for closing are addressed -
There is a test suite reasonably covering new functionality or modifications -
Function naming, parameters, return values, types, etc., are consistent and according to CONTRIBUTION.md
-
This feature/change has adequate documentation added -
No obvious mistakes in the code