Fix build warning for use of `strdup` in ultrajson
When building with `-std=c99` (I was having a look at @lithomas1's `meson-poc` branch) on Linux this gives a warning:

```
[2/3] Compiling C object pandas/_libs/json.cpython-310-x86_64-linux-gnu.so.p/src_ujson_lib_ultrajsonenc.c.o
../pandas/_libs/src/ujson/lib/ultrajsonenc.c: In function 'JSON_EncodeObject':
../pandas/_libs/src/ujson/lib/ultrajsonenc.c:1180:18: warning: implicit declaration of function 'strdup'; did you mean 'strcmp'? [-Wimplicit-function-declaration]
 1180 |         locale = strdup(locale);
      |                  ^~~~~~
      |                  strcmp
../pandas/_libs/src/ujson/lib/ultrajsonenc.c:1180:16: warning: assignment to 'char *' from 'int' makes pointer from integer without a cast [-Wint-conversion]
 1180 |         locale = strdup(locale);
```

And for the MSVC builds in CI the warnings look like:
```
building 'pandas._libs.json' extension
creating build\temp.win-amd64-cpython-310\Release\pandas\_libs\src\ujson
creating build\temp.win-amd64-cpython-310\Release\pandas\_libs\src\ujson\lib
creating build\temp.win-amd64-cpython-310\Release\pandas\_libs\src\ujson\python
"C:\Program Files\Microsoft Visual Studio\2022\Enterprise\VC\Tools\MSVC\14.33.31629\bin\HostX86\x64\cl.exe" /c /nologo /O2 /W3 /GL /DNDEBUG /MD -DNPY_NO_DEPRECATED_API=0 -Ipandas/_libs/src/ujson/python -Ipandas/_libs/src/ujson/lib -Ipandas/_libs/src/datetime -IC:\Users\runneradmin\micromamba\envs\test\lib\site-packages\numpy\core\include -IC:\Users\runneradmin\micromamba\envs\test\include -IC:\Users\runneradmin\micromamba\envs\test\Include "-IC:\Program Files\Microsoft Visual Studio\2022\Enterprise\VC\Tools\MSVC\14.33.31629\include" "-IC:\Program Files\Microsoft Visual Studio\2022\Enterprise\VC\Tools\MSVC\14.33.31629\ATLMFC\include" "-IC:\Program Files\Microsoft Visual Studio\2022\Enterprise\VC\Auxiliary\VS\include" "-IC:\Program Files (x86)\Windows Kits\10\include\10.0.22621.0\ucrt" "-IC:\Program Files (x86)\Windows Kits\10\\include\10.0.22621.0\\um" "-IC:\Program Files (x86)\Windows Kits\10\\include\10.0.22621.0\\shared" "-IC:\Program Files (x86)\Windows Kits\10\\include\10.0.22621.0\\winrt" "-IC:\Program Files (x86)\Windows Kits\10\\include\10.0.22621.0\\cppwinrt" "-IC:\Program Files (x86)\Windows Kits\NETFXSDK\4.8\include\um" /Tcpandas/_libs/src/ujson/lib/ultrajsondec.c /Fobuild\temp.win-amd64-cpython-310\Release\pandas/_libs/src/ujson/lib/ultrajsondec.obj -D_GNU_SOURCE
ultrajsondec.c
pandas/_libs/src/ujson/lib/ultrajsondec.c(1178): warning C4996: 'strdup': The POSIX name for this item is deprecated. Instead, use the ISO C and C++ conformant name: _strdup. See online help for details.
```

I considered avoiding the code duplication by putting this block in a shared file, but the two files touched here already have quite a bit of duplicated code, so that's best left alone.
On Linux, this should be provided if `_XOPEN_SOURCE` is defined to at least 500, or `_POSIX_C_SOURCE` to at least 200809L (when string.h is included). > On Linux, this should be provided if `_XOPEN_SOURCE` is defined to at least 500, or `_POSIX_C_SOURCE` to at least 200809L (when string.h is included).

Thanks @eli-schwartz. That's an alternative indeed, and I did consider it - but not sure if that is better / more portable? It's certainly more portable, it's a POSIX function and that's the POSIX define that is supposed to make it accessible: https://pubs.opengroup.org/onlinepubs/9699919799/functions/V2_chap02.html#tag_15_02

The advantage is that instead of telling the compiler "I pinky promise the system runtime library actually provides this", you tell the system runtime headers to actually provide it. If it still doesn't provide it, then you're probably on Windows, in which case the _MSC_VER workaround is apropos, or else something probably went wrong and you're delaying a compiler diagnostic / error to a linker error. Great thanks for this - always good to clean these up. We already define some portability items in pandas/_libs/src/headers/portable.h - maybe worth placing there instead? Updated to use `_XOPEN_SOURCE` (600 rather than 500, which seems necessary with XCode 12 on macOS) and moved to `portable.h` as suggested. Thanks for the PR. Awesome thanks a ton @rgommers  Thanks for the reviews & merge. And yay, my first Pandas PR (long overdue) 🎉 