Use _MSVC_LANG macro when compiled with MSVC for C++ standard detection

[fktn-k]

Although this library depends on C++11 by default, some implementations are switched based on the value of the __cplusplus macro to utilize some nice-to-have features available since C++14 or better.
However, the MSVC compilers don't set an appropriate value to that macro if the /Zc:__cplusplus option isn't specified.
That issue forces those who use this library with some MSVC compiler to stick to C++11 features even if they explicitly use C++14 or better by specifying options like /std:c++14.

As explicitly described here, the _MSVC_LANG macro is always set a correct value.
So, as many other C++ libraries do, this library refers to _MSVC_LANG if some MSVC compiler is used to compile, and now detects a correct C++ standard.
The updated behavior is tested in my local environment (Windows10, Visual Studio 2019).

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• [x] Changes are described in the pull request or in a referenced issue.
• [x] The test suite compiles and runs without any error.
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Please don't

• The C++11 support varies between different compilers and versions. Please note the list of supported compilers. Some compilers like GCC 4.7 (and earlier), Clang 3.3 (and earlier), or Microsoft Visual Studio 13.0 and earlier are known not to work due to missing or incomplete C++11 support. Please refrain from proposing changes that work around these compiler's limitations with #ifdefs or other means.
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• Please do not open pull requests that address multiple issues.

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