Conditional Compilation for C++17 Compatibility

[iconnor]

This pull request wraps two specific files in a conditional compilation block to allow builds using the C++17 standard. The goal is to enhance compatibility without affecting existing functionality.

Motivation: I am utilizing this library in a PlatformIO project that uses C++17 for both native CI/CD and testing purposes. While I understand that the library aims to use the latest C++ standards, there are scenarios where projects may need to use older but still widely-used standards like C++17.

Changes: Wrapped the offending code in #if __cplusplus >= 202002L ... #endif blocks to allow compilation under C++17. Benefits: Increases the library's compatibility with projects that are not yet able to migrate to C++20. Allows for more diverse usage in larger codebases that may be using multiple C++ standards. No impact on existing functionality when used in a C++20 environment. Alternative Considered: An alternative could be to completely exclude these files during the build process, but conditional compilation offers a less invasive approach.

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Pull request checklist

Read the Contribution Guidelines for detailed information.

• [ ] Changes are described in the pull request, or an existing issue is referenced.
• [ ] The test suite compiles and runs without error.
• [ ] Code coverage is 100%. Test cases can be added by editing the test suite.
• [ ] The source code is amalgamated; that is, after making changes to the sources in the include/nlohmann directory, run make amalgamate to create the single-header files single_include/nlohmann/json.hpp and single_include/nlohmann/json_fwd.hpp. The whole process is described here.

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.
• Specifically, I am aware of compilation problems with Microsoft Visual Studio (there even is an issue label for this kind of bug). I understand that even in 2016, complete C++11 support isn't there yet. But please also understand that I do not want to drop features or uglify the code just to make Microsoft's sub-standard compiler happy. The past has shown that there are ways to express the functionality such that the code compiles with the most recent MSVC - unfortunately, this is not the main objective of the project.
• Please refrain from proposing changes that would break JSON conformance. If you propose a conformant extension of JSON to be supported by the library, please motivate this extension.
• Please do not open pull requests that address multiple issues.

[gregmarr]

This library is actually 100% C++11 compatible. It can take advantage of later standards if you compile with them, but it doesn't require anything later than that.

The files you modified are examples used in the documentation, not part of the library itself.

[nlohmann]

What your code is proposing is already done in the library: any feature that is not pure C++11 is guarded by macros. The MR like this makes no sense, because the example files are just documentation.