* gio/src/cancellable.ccg: Add TODO comment.
* glib/glibmm/class.cc: Call custom_class_base_finalize_function() and
custom_class_init_function() via local functions with C linkage.
* glib/glibmm/extraclassinit.h: Point out in the class documentation that
the class init and instance init functions shall have C linkage.
* glib/glibmm/main.[cc|h]: Call prepare_vfunc(), check_vfunc() and
dispatch_vfunc() via local functions with C linkage.
* glib/glibmm/objectbase.cc: Call destroy_notify_callback()
via a local function with C linkage.
* glib/glibmm/propertyproxy_base.cc: Call PropertyProxyConnectionNode::
callback() and destroy_notify_handler() via local functions with C linkage.
* glib/glibmm/signalproxy.cc: Call SignalProxyNormal::slot0_void_callback()
and SignalProxyConnectionNode::destroy_notify_handler() via local functions
with C linkage.
* glib/src/binding.ccg: Add extern "C".
* glib/src/bytearray.ccg: Add a TODO comment.
* glib/src/markup.ccg: Call functions in the vfunc table via local
functions with C linkage.
* glib/src/optioncontext.ccg: Add extern "C".
* glib/src/optiongroup.ccg: Call post_parse_callback() and
option_arg_callback() via local functions with C linkage.

Part of issue #1