Additional fix std::filesystem with /std:c++17

[ipoliop-8]

The change in #1058 was incomplete as this constant is checked in two places. Fix it in the second one as well.

• [x] Fixes #1059 i.e. actually fixes #1056

[jdonald]

@shanshan0309 could you test this out? Sorry that it has taken too many iterations to get this right.

[shanshan0309]

@jdonald. Thank you. I tried your fix and it works well.