Function relativeIfUnder

Source/cmLocalGenerator.cxx:4338–4349

namespace {
std::string relativeIfUnder(std::string const& top, std::string const& cur,
                            std::string const& path)
{
  // Use a path relative to 'cur' if it can be expressed without
  // a `../` sequence that leaves 'top'.
  if (cmSystemTools::IsSubDirectory(path, cur) ||
      (cmSystemTools::IsSubDirectory(cur, top) &&
       cmSystemTools::IsSubDirectory(path, top))) {
    return cmSystemTools::ForceToRelativePath(cur, path);
  }
  return path;
}
}

std::string cmLocalGenerator::GetRelativeSourceFileName(