Windows compile error, and fix

[lgritz]

In the current master, format.h:312,

if (const_check(sizeof(isinf(x)) != sizeof(dummy_int)))

I'm seeing an error about ambiguous use of dummy_int on Windows (MSVS 14.0, I think).
I believe that the fix is just to change it to fmt::internal::dummy_int, as it is a few lines later.

I'm very sorry about reporting it like this rather than submitting a PR. I maintain large open source projects myself and it drives me crazy when people do this. But the fact is that I work in a large company where it's a considerable hassle to get legal approval to contribute a PR containing actual code to a project that hasn't already been vetted. It would take me weeks, especially over the holiday season. Sorry again.

[vitaut]

Committed a tentative fix in 01640f4. Let me know if it resolves the problem and thanks for reporting!

[lgritz]

Yes, works! Thanks.