fixed TLS for Win8.1 SChannel

Windows 8.1 disables the support for RC4 in TLS by default,
and libstrophe fails to connect with "Couldn't start TLS! error -2146893048"

Since it's impossible to tell Win8.1 from Win8.0 with GetVersion(), unless the app is manifested for Win8.1, the fix enables CALG_AES for Win8.0+.