Fix extern declaration of default_factory with initializer
Fix extern declaration of default_factory with initializer

This declaration of default_factory as extern, but with an initializer,
elicits a warning when compiled using clang.  This declaration is on the
MSVC codepath, but the warning fires when library is built for Windows
using clang-cl.

google/cctz,Jellybooks/cctz,google/cctz,Jellybooks/cctz,Jellybooks/cctz