Linker issue with SSL support

[TheJosh]

src/ftp/gnutls-socket.o: In function 'sock_ssl_create_impl':
gnutls-socket.c:312: undefined reference to 'gnutls_certificate_set_verify_function'

I have GNU TLS 2.8.5 (Ubuntu 10.04 LTS), but that function was only added in 2.10.0:
http://manpages.courier-mta.org/htmlman3/gnutls_certificate_set_verify_function.3.html

[sebastinas]

At some point I had >= 3.x as required version but I think this should wait until the SSL support is finished. Especially the certificate handling has many more convenience function in later GnuTLS releases.