g_value_set_{enum,flags}() always assigns value to v_(u)long but if
G_ENABLE_DEBUG is not set, the value is read from v_(u)int instead.  This
apparently breaks on 64-bit architectures.  See bug 144370 for symptoms.

I'm not exactly sure if genums.c is equally broken (int vs. long) but somethings
clearly not right here.

#ifdef G_ENABLE_DEBUG
...
#define g_marshal_value_peek_enum(v)     g_value_get_enum (v)
#define g_marshal_value_peek_flags(v)    g_value_get_flags (v)
...
#else
...
#define g_marshal_value_peek_enum(v)     (v)->data[0].v_int
#define g_marshal_value_peek_flags(v)    (v)->data[0].v_uint
...
#endif

and in gobject/genums.c:

gint
g_value_get_enum (const GValue *value)
{
  g_return_val_if_fail (G_VALUE_HOLDS_ENUM (value), 0);

  return value->data[0].v_long;
}

guint
g_value_get_flags (const GValue *value)
{
  g_return_val_if_fail (G_VALUE_HOLDS_FLAGS (value), 0);

  return value->data[0].v_ulong;
}