Avoid warnings from -Wmissing-prototypes -Wundef -Wunused-label

-Wmissing-prototypes was complaining about declaring XS() functions without previously declaring a prototype.

-Wundef didn't like using #if foo instead of #ifdef foo

-Wunused-label warned because VER_{IV,NM,PV} were defined on all versions of perl, but only used on < 5.17.2