Log Message:
(math/R-xfun) Updated 0.25 to 0.26

# CHANGES IN xfun VERSION 0.26

- The `windows_only` argument of `native_encode()` has been
  removed. Now `native_encode()` only tries the conversion to native
  encoding on platforms where `l10n_info()[['UTF-8']]` does not return
  `TRUE`.

- Added a `solaris` argument to `upload_win_builder()`.