Fix __cplusplus macro for Visual Studio

[Twon]

So it turns out that Visual Studio has been incorrectly defining the __cplusplus macro all along, and so checks which depend on this, just as memory resource tests, are incorrectly not executed on Windows.

This is described in the following:

https://stackoverflow.com/a/56483887/4764531
https://devblogs.microsoft.com/cppblog/msvc-now-correctly-reports-__cplusplus/
The compiler option /Zc:__cplusplus ensures Visual Studio update the value of this macro.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 96.96%. Comparing base (d6f7272) to head (a381d8a).
Report is 3 commits behind head on main.

❗ Current head a381d8a differs from pull request most recent head 251e704. Consider uploading reports for the commit 251e704 to get more accurate results

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #237      +/-   ##
==========================================
- Coverage   97.00%   96.96%   -0.05%     
==========================================
  Files         131      126       -5     
  Lines        4171     4115      -56     
==========================================
- Hits         4046     3990      -56     
  Misses        125      125              

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[Twon]

The build failures here are caused not by Catch2 this time, but a similar issue with GTest. Conan is pulling down packaged build with C++14 from the Conan Index Centre resulting in this issue: google/googletest#3081

[Twon]

Issue identified in Conan Gtest package: conan-io/conan-center-index#22547

A fix should be able to soon go into Conan 2.1

[Twon]

Issue identified in Conan Gtest package: conan-io/conan-center-index#22547

A fix should be able to soon go into Conan 2.1

It turns out this does not fix the issues which is actually affecting Windows, which is that Conan does see any binary compatibility between the C++17 and C++20 versions which there is. This is a know issue for Conan, and requires a fix is committed to the GTest recipe and is the same issue affecting Catch2 currently: #241

[Twon]

I've now sepperated the updating the GTest package sync to rely on pre-built packages to a separate PR so the fix to Visual Studio C++ flag can go in now: #291