Warn when classes/instances exceed the allowed size, if possible

It's impossible to check this if the library user is using
g_type_register_static, but in that case their compiler should hopefully
warn about the truncation. This fixes it for G_DEFINE_TYPE and friends,
at least.

Bug: https://bugzilla.gnome.org/show_bug.cgi?id=659916


Signed-off-by: Simon McVittie <simon.mcvittie@collabora.co.uk>
Reviewed-by: Colin Walters <walters@verbum.org>