add SPENGO support

[Tagar]

Please build libcurl/curl with SPNEGO support so we can make requests to Kerberos-enabled services.

Options that curl was built with (from curl -V output)
from RHEL 7.6 as an example:

Features: AsynchDNS GSS-Negotiate IDN IPv6 Largefile NTLM NTLM_WB SSL libz unix-sockets

from Anaconda:

Features: AsynchDNS IPv6 Largefile NTLM NTLM_WB SSL libz TLS-SRP UnixSockets HTTPS-proxy

Missing libcurl options build option to add GSS-Negotiate support:

GSSAPI support: no (--with-gssapi) 
SNPEGO support: no (--with-spnego)

Thank you.