media: fix relay-info with Farstream 0.2

Type of "relay-info" property changed from GValueArray to GPtrArray in
Farstream 0.2. Update our purple media backend accordingly when building
with libpurple 3.x.x.
configure: add option for libpurple 3

Build can now be configured with --enable-purple=3 to enable compilation
against libpurple 3 and GStreamer 1.0.