fix: compilation error with libc++

[ItsAlbertZhang]

I use Crow on Windows, MacOS, and Linux. On Windows, I compile with the MSVC compiler, while on MacOS and Linux, I use the Clang compiler. It works well.

The issue arose when migrating from my local development environment on Linux (WSL, Ubuntu Preview 24.04 daily live) to a cloud server (Debian 12). Since Debian 12 lacks the latest Clang compiler and C++ Library, I had to install libc++ as a replacement for libstdc++, which led to compilation problems. Subsequently, linking the version that ran correctly on WSL to libc++ instead of libstdc++ also resulted in the same issue.

After searching for relevant information in this repository, I found that issues #229, #481, and #636, as well as pull requests #229 and #634, might be related. After modifying json.h, my code compiled and ran successfully.

I'm unsure if _LIBCPP_VERSION would be defined under defined(__APPLE__) || defined(__MACH__) || defined(__FreeBSD__) || defined(__ANDROID__), so I didn't remove the conditional checks in the original code. Instead, I added checks for _LIBCPP_VERSION directly after them. If possible, you can further investigate this.

I'm a beginner in C++, and English is not my first language, so please forgive any inaccuracies in my wording.

[gittiver]

Looks Ok, I will add an issue for compiling with gcc and clang as matrix build for getting these variants checked

[ItsAlbertZhang]

Looks Ok, I will add an issue for compiling with gcc and clang as matrix build for getting these variants checked

It seems more like an issue with the C++ Library rather than the compiler itself. In my case, Clang works well with libstdc++, but encounters this compilation error when working with libc++.