It seems that G++ 8 is getting more strict about casting function pointers, and this shows up in a warning spew for every G_DEFINE_TYPE because of the unused class_data argument to GClassInitFunc and GInstanceInitFunc:

From, e.g. https://gitlab.gnome.org/GNOME/gjs/-/jobs/6475:

```
In file included from /usr/include/glib-2.0/gobject/gobject.h:24,
                 from /usr/include/glib-2.0/gobject/gbinding.h:29,
                 from /usr/include/glib-2.0/glib-object.h:23,
                 from /usr/include/glib-2.0/gio/gioenums.h:28,
                 from /usr/include/glib-2.0/gio/giotypes.h:28,
                 from /usr/include/glib-2.0/gio/gio.h:26,
                 from /root/jhbuild/checkout/gjs/gjs/context.cpp:32:
/root/jhbuild/checkout/gjs/gjs/context.cpp: In function 'GType gjs_context_get_type()':
/root/jhbuild/checkout/gjs/gjs/context.cpp:123:27: warning: cast between incompatible function types from 'void (*)(gpointer)' {aka 'void (*)(void*)'} to 'GClassInitFunc' {aka 'void (*)(void*, void*)'} [-Wcast-function-type]
 G_DEFINE_TYPE(GjsContext, gjs_context, G_TYPE_OBJECT);
                           ^~~~~~~~~~~
/usr/include/glib-2.0/gobject/gtype.h:1977:57: note: in definition of macro '_G_DEFINE_TYPE_EXTENDED_BEGIN_REGISTER'
                                        (GClassInitFunc) type_name##_class_intern_init, \
                                                         ^~~~~~~~~
/usr/include/glib-2.0/gobject/gtype.h:1737:60: note: in expansion of macro '_G_DEFINE_TYPE_EXTENDED_BEGIN'
 #define G_DEFINE_TYPE_EXTENDED(TN, t_n, T_P, _f_, _C_)     _G_DEFINE_TYPE_EXTENDED_BEGIN (TN, t_n, T_P, _f_) {_C_;} _G_DEFINE_TYPE_EXTENDED_END()
                                                            ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~
/usr/include/glib-2.0/gobject/gtype.h:1595:43: note: in expansion of macro 'G_DEFINE_TYPE_EXTENDED'
 #define G_DEFINE_TYPE(TN, t_n, T_P)       G_DEFINE_TYPE_EXTENDED (TN, t_n, T_P, 0, {})
                                           ^~~~~~~~~~~~~~~~~~~~~~
/root/jhbuild/checkout/gjs/gjs/context.cpp:123:1: note: in expansion of macro 'G_DEFINE_TYPE'
 G_DEFINE_TYPE(GjsContext, gjs_context, G_TYPE_OBJECT);
 ^~~~~~~~~~~~~
/root/jhbuild/checkout/gjs/gjs/context.cpp:123:27: warning: cast between incompatible function types from 'void (*)(GjsContext*)' {aka 'void (*)(_GjsContext*)'} to 'GInstanceInitFunc' {aka 'void (*)(_GTypeInstance*, void*)'} [-Wcast-function-type]
 G_DEFINE_TYPE(GjsContext, gjs_context, G_TYPE_OBJECT);
                           ^~~~~~~~~~~
/usr/include/glib-2.0/gobject/gtype.h:1979:60: note: in definition of macro '_G_DEFINE_TYPE_EXTENDED_BEGIN_REGISTER'
                                        (GInstanceInitFunc) type_name##_init, \
                                                            ^~~~~~~~~
/usr/include/glib-2.0/gobject/gtype.h:1737:60: note: in expansion of macro '_G_DEFINE_TYPE_EXTENDED_BEGIN'
 #define G_DEFINE_TYPE_EXTENDED(TN, t_n, T_P, _f_, _C_)     _G_DEFINE_TYPE_EXTENDED_BEGIN (TN, t_n, T_P, _f_) {_C_;} _G_DEFINE_TYPE_EXTENDED_END()
                                                            ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~
/usr/include/glib-2.0/gobject/gtype.h:1595:43: note: in expansion of macro 'G_DEFINE_TYPE_EXTENDED'
 #define G_DEFINE_TYPE(TN, t_n, T_P)       G_DEFINE_TYPE_EXTENDED (TN, t_n, T_P, 0, {})
                                           ^~~~~~~~~~~~~~~~~~~~~~
/root/jhbuild/checkout/gjs/gjs/context.cpp:123:1: note: in expansion of macro 'G_DEFINE_TYPE'
 G_DEFINE_TYPE(GjsContext, gjs_context, G_TYPE_OBJECT);
 ^~~~~~~~~~~~~
```

I realize all of GObject depends heavily on this technically undefined behaviour, and that's not going to change, so maybe we should disable this warning around gtype.h?

The attached patch is untested. It's an alternative to disabling the warning, but requires all client code to add a gpointer parameter to their instance_init functions, so probably not an option.