We can't initialise gslice from a ctor because g_slice_set_config() must
be called before gslice initialisation.

Instead, do the initialisation in a threadsafe way from the
initialisation function for the thread private data.  This will only be
called once per thread so the synchronisation doesn't pose a significant
overhead here.

Ensure that we try to grab the thread private data directly on entrance
to g_slice_alloc() so that we force the initialisation to occur.
Grabbing the private data is the common case anyway.

https://bugzilla.gnome.org/show_bug.cgi?id=660887