Remove check for anonymous unions from configure and cmake config

Remove check for anonymous unions from configure and cmake config

Anonymous unions/structs are technically a custom GNU C99 feature but
was already widely supported by other compilers. With C11 this feature
has become a standard feature so all compilers nowadays support it.

Change-Id: I1ef5f6f21f0135a628a63553c39515fa4549ce87
Signed-off-by: Arne Schwabe <arne@rfc2549.org>
Acked-by: Frank Lichtenheld <frank@lichtenheld.com>
Message-Id: <20240710160238.190189-1-frank@lichtenheld.com>
URL: https://www.mail-archive.com/openvpn-devel@lists.sourceforge.net/msg28914.html
Signed-off-by: Gert Doering <gert@greenie.muc.de>