macos: use realpath$DARWIN_EXTSN
macOS 10.15 Catalina causes an ancient variant of `realpath` to fail,
likely due to 10.15's evolving security model.
closes #5325
e05923f34bMichael Dusan committed on 5/16/2020, 4:41:53 PM· committed by Andrew Kelleyparent79462bb