Fix ssize_t on Windows

[ales-tsurko]

Rust's isize converted into ssize_t, but it's not defined on Windows. That leads to compilation errors. There's SSIZE_T though.

[dtolnay]

Thanks!

Were you able to confirm that this fixes your problem? It looks like it doesn't to me, since this header does not get included by the generated cc file.

[myronahn]

I have a similar fix in my pull request. It does seem to fix the problem although it could be that the header just happened to be included in all the right places.

15b5b10

[dtolnay]

Ah I guess this relies on at least one of the include!'d files to contain #include "rust/cxx.h", which wouldn't always be the case.

Is there a meaningful difference between using _WIN32 vs _MSC_VER?

Alternatively we could do this based on env::var_os("CARGO_CFG_WINDOWS").is_some() for Cargo builds since Cargo guarantees that is passed for all Windows targets, and use something like a --windows flag for the cxxbridge cli command for the case of non-Cargo builds.

[dtolnay]

I think I prefer @myronahn's use of a type alias instead of a macro for this. But I would prefer not to have our header define ssize_t in the root namespace. Could we define it as rust::isize instead, using ssize_t as the type in the non-Windows case?

[dtolnay]

I've published cxx 0.2.7 containing the fix from #97.