specify c++ standard flag for msvc

[philipcraig]

No description provided.

[philipcraig]

I can see that it's only the four builds on nightly that are failing, and I can reproduce this locally on nightly. stable is fine.

Note that the failed build is not because of trying to include the Windows flag support, but by trying to push the C++ version feature flags so that they are visible to the test suite's build.rs command. I guess as a workaround we could force the test suite to always use a particular C++ version and not worry about allowing a C++ version feature for the test suite?

I'm not clear if it's a bug/stricter nightly requirement or a nightly regression though. Tempted to leave this open a few days and see if nightly fixes this.

It looks like another instance of rust-lang/cargo#6941, which leads to an open bug raised by @dtolnay here rust-lang/cargo#6915

The cargo bugs mention that part of the issue may be that cxx-test-suite is in dev-dependencies

[dtolnay]

I implemented an alternate solution in #268 which avoids the Cargo dependency resolution bug. Thanks anyway!