Regex doesn't compile under clang-cl with warnings-as-errors=on

[pdimov]

E.g. https://github.com/boostorg/unordered/actions/runs/10655219740/job/29532497175?pr=277#step:6:489

compile-c-c++ bin.v2\libs\regex\build\clng-win-16.0.\dbg\x86_3\cxstd-14-iso\inln-on\optmz-spc\thrd-mlt\wide_posix_api.obj
C:\actions-runner\_work\unordered\boost-root\libs\regex\build\..\src\wide_posix_api.cpp(199,21): error: 'wcscpy' is deprecated: This function or variable may be unsafe. Consider using wcscpy_s instead. To disable deprecation, use _CRT_SECURE_NO_WARNINGS. See online help for details. [-Werror,-Wdeprecated-declarations]
               std::wcscpy(buf, localbuf);
                    ^

This wasn't a problem before the modular changes because Serialization is referenced with <warnings>off here

https://github.com/boostorg/unordered/blob/21937249c47da1fbdb445dc40775a88832fd772d/test/Jamfile.v2#L156

However, post the modular changes Serialization depends on Regex, and <warnings>off doesn't get propagated to the Regex build for some reason, resulting in the error above.

I see that a PR addressing this (#218) has already been merged, but it doesn't seem to help for some reason.

The preprocessor condition should probably check for the MS STL, rather than specific compilers.

[pdimov]

Or maybe the file should just define _CRT_SECURE_NO_WARNINGS at the top and dispense with the #ifs.

[pdimov]

#220 implements the first suggested fix. I'll make a second PR for the second.

[pdimov]

#221 implements the second suggested fix.

I've verified locally that both work.