Cover invalid 'if' in struct members, features, union and alternate
branches.  Four out of four are broken.  Mark FIXME.
Signed-off-by: NMarkus Armbruster <armbru@redhat.com>
Message-Id: <20190914153506.2151-6-armbru@redhat.com>
Reviewed-by: NEric Blake <eblake@redhat.com>
[Comment typo fixed]