Vendor libgss when static-curl and spnego features are set

[hhirtz]

For spnego authentication to work, curl needs to be built with "a suitable GSS-API library or SSPI on Windows" [0].

Right now, the build fails if static-curl and spnego features are set, and no suitable lib is found. What do you think of vendoring this dependency in this case?

This is for proxy authentication support in cargo:
rust-lang/cargo#7330
rust-lang/cargo#11433

[0] https://docs.rs/curl/0.4/curl/easy/struct.Auth.html#method.gssnegotiate