Incorrect linkage for C++ functions called from C

In hudclient.cpp, we have:

static void loadingCB(GObject* /*src*/, gpointer dst)
{
    static_cast<HudClient*>(dst)->voiceQueryLoading();
}

// ...

 g_signal_connect(G_OBJECT(m_clientQuery), "voice-query-loading", G_CALLBACK(loadingCB), this);

The loadingCB is passed to GLib as a callback. This is incorrect as it stands. Declaring a function static only controls its visibility, but not its linkage. It happens to work with gcc, but that's coincidental; on other platforms, this may cause a crash.

To specify C linkage, an extern "C" directive is needed. Also, there is no need for the static keyword; instead, the relevant functions can be placed into an anonymous namespace.