As of f69354d9fa, this is just always
expected on all platforms. If it's not defined,
TemplateClass<X>::_type_handle won't link correctly with some compilers
(e.g. Clang) and the lack of 'extern template class' will result in
duplicated code in any case. In short, turning it off causes problems.
makepanda also always turns this on, so it doesn't represent any
optional feature or autodetected platform-specific behavior anyway.