mariadb-backup can't use gssapi authentication on Windows

This issue was initially reported by Dlanod Kcud on Stackoverflow

With implementation of MDEV-26715 gssapi client authentication plugin became static on Windows and is no longer available as a .dll library. Since mariadb-backup doesn't link against Connector/C but against embedded server library (libmysqld) gssapi authentication doesn't work anymore.

Possible solutions:

• When building C/C inside server, build static auth plugins also as shared plugins
• Link static auth plugins against libmysqld.