[Folly] Disable non-underscore posix names on windows

[PhoebeHui]

Fix related issue #6670.

Background:
In previous windows sdk verison 10.0.17763.0, we used to do the equivalent of checking for STDC on a per-file basis with:
#define _CRT_INTERNAL_NONSTDC_NAMES
(
( defined _CRT_DECLARE_NONSTDC_NAMES && _CRT_DECLARE_NONSTDC_NAMES) ||
(!defined _CRT_DECLARE_NONSTDC_NAMES && !STDC )
)

Then in individual files, doing
#if _CRT_INTERNAL_NONSTDC_NAMES
// define mktemp, chmod, close, etc
#endif // _CRT_INTERNAL_NONSTDC_NAMES

However, that takes advantage of non-standard preprocessor behavior, in windows sdk 10.0.18362.0 we switched to this:
#if ( defined _CRT_DECLARE_NONSTDC_NAMES && _CRT_DECLARE_NONSTDC_NAMES) ||
(!defined _CRT_DECLARE_NONSTDC_NAMES && !STDC )
#define _CRT_INTERNAL_NONSTDC_NAMES 1
#else
#define _CRT_INTERNAL_NONSTDC_NAMES 0
#endif

So, we’re no longer evaluating STDC on a per-file basis, but on a whole project basis.

[ayermolo]

@PhoebeHui I am trying to build VS 2017 project that links against folly, and running in to error:
googletest\googletest\include\gtest/internal/gtest-port.h(2025): error C3861: 'chdir': identifier not found.
Because of #define _CRT_INTERNAL_NONSTDC_NAMES 0 in Windows.h.

Anything I am missing?
Thank You