Part 2 of my attempt to run the [Niko Home Control programming software](https://appdb.winehq.org/objectManager.php?sClass=application&iId=21635) under Wine. This .NET application ends up calling \``PFXExportCertStoreEx` (or `PFXExportCertStore` ) since it redirects to it which is a stub at the moment. According to the [documentation](https://learn.microsoft.com/en-us/windows/win32/api/wincrypt/nf-wincrypt-pfx...), if the `EXPORT_PRIVATE_KEYS` flag is set, we should export the private key as well. For this, we use `NCryptExportKey` which is part of `ncrypt` so we have to add `ncrypt` to the `Makefile.in` as well. For exporting, I used GnuTLS library. As it seems to be only available in the unix side, I added a helper in `unixlib.c`. **EDIT** Looking at the behavior of Windows with the tests, it seems that when `cert` is `NULL` we should not fail but instead produce a valid but empty `PKCS#12`. I changed this to reflect Windows' behavior. -- v6: Use advice from review comment https://gitlab.winehq.org/wine/wine/-/merge_requests/10532