libvncserver: openssl support needs pthreads

Fixes http://autobuild.buildroot.net/results/c30/c3082693fe0da0c54d4bbf950dd6d74e1395c1d9/



Also pass --without-ssl if that isn't the case, as the configure script
otherwise ends up detecting openssl if built before libvncserver.

Signed-off-by: Peter Korsgaard <peter@korsgaard.com>