Yes! That fixed my issue! Previously I thought I had tried to compile with CXX_EXTENSIONS ON, but apparently not. That seems to be equivalent to -std=gnu++1y.
Side note, if I did need my flag (~-std=gnu++1y) to be at the end, behind the CXX_STANDARD` flag, I’m not sure how I would’ve accomplished this. This issue seems to relate to my struggle, https://gitlab.kitware.com/cmake/cmake/issues/16608. Though my issue is solved 
, thank you!