Fix 2 bugs with Language Standard Feature (#1317) * Fix setting `HXCPP_CPP__` not working on `Build.xml`. * Fix language standards not being remembered from parent file groups. * [tests] Add tests for c/cpp standard regressions * [ci] Use bash for cxx standard for loop --------- Co-authored-by: Tobiasz Laskowski <tobil4sk@outlook.com>