pidgin: build with cyrus_sasl for SASL in IRC and XMPP

SASL is required to connect to Freenode on high-abuse networks, e.g. highly
NATed networks or Tor. However, Pidgin only supports PLAIN authorization type,
and Freenode specifically prohibits using anything other than EXTERNAL or
ECDSA-NIST256P-CHALLENGE over Tor. It still allows PLAIN for potentially
problematic clearnet networks, though.