Fix debug builds under MSVC
Fix debug builds under MSVC

The C spec leaves conditional evaluation inside a macro expansion as
undefined behaviour. This means we cannot use constructs like:

  GOBJECT_IF_DEBUG(OBJECTS, {
    ...
  #ifdef BLAH
    ...
  #endif
    ...});

Because compilers are entirely justified to ignore the conditional, or,
like in the case of MSVC, error out.

https://bugzilla.gnome.org/show_bug.cgi?id=769504

johne53/MB3Glib,johne53/MB3Glib,endlessm/glib,johne53/MB3Glib,johne53/MB3Glib,johne53/MB3Glib,endlessm/glib,johne53/MB3Glib,endlessm/glib,endlessm/glib,endlessm/glib