Several methods no longer exist. Patching just the pkg-config file doesn't suffice this time. Implicit declarations lead to the plugin failing to load.
From 0.10 on, libchamplain needs GTK+ 3. As GTK+ 2 and 3 cannot be mixed in the same process, support for libchamplain >= 0.10 is only possible once Claws Mail works with GTK+ 3.
Confirmed. I've made the same discovery meanwhile. Thanks for the quick reply, Holger!
http://git.claws-mail.org/?p=claws.git;a=commit;h=3c64fb648dbb155c971fd0108f40fee7a36d2cb1