[build2] Constraining the C++ standard

[lyrahgames]

Hello,

for some testing, I have package the cxxopts library.

https://github.com/lyrahgames/build2-cxxopts

The tests for this library are based on the old version 1.5.0 of Catch.
Unfortunately, this version does not compile with the latest C++ 
standard enabled.
Hence, in contrast to the recommendation in the build system manual,
I constrained the standard to C++14 with 'cxx.std = c++14' in the 
'root.build' of the tests package.
The header-only library package itself still sets 'cxx.std = latest' to 
not constrain anything else.
As the tests will never be a dependency of any other project, I thougt 
this should be ok.
And indeed, on my computer as for the GCC- and Clang-based CI builds, 
it works without problems.
However, building on Windows with MSVC the builds fail already in the 
configuration step
with the compiler complaining about an unsupported C++ standard 
'C++c++14':
"error: C++c++14 is not supported by Microsoft (R) C/C++ Optimizing 
Compiler Version 19.36.32534 for x64"

I assume that the specific standards are not mapped to the compilers.
Is there any way for me to constrain the standard consistently for all 
compilers?
Or should I deal with this problem in a completely different manner?

Also, what should I do if a library that is not binless only compiles 
with a specific older standard
but not with the latest one, as it is for ChaiScript:

https://github.com/build2-packaging/chaiscript

As I got it from the manual, there might be ABI-breaking changes 
between standards.
For now, I constrained the standard to C++17 for ChaiScript as it was 
the only thing I could think of,
still, constraining the builds to GCC and Clang and possibly breaking 
ABI across different projects.

Thank you very much in advance for your help.

Greetings
Markus