Build fails to pass `-std=c99` flag: `blosc2.h: error: ‘for’ loop initial declaration used outside C99 mode`

`blosc2` itself sets `-std=gnu99` in its own CMakeLists, however those are not used by `py-blosc2` when external `blosc2` is linked against. So the flag is not passed, but it is required:

```
In file included from /opt/local/var/macports/build/_opt_PPCSnowLeopardPorts_python_py-blosc2/py311-blosc2/work/blosc2-2.5.1/_skbuild/macosx-10.6-ppc-3.11/cmake-build/blosc2/blosc2_ext.c:1253:
/opt/local/include/blosc2.h: In function ‘blosc2_meta_exists’:
/opt/local/include/blosc2.h:2162: error: ‘for’ loop initial declaration used outside C99 mode
```

检查在链接外部 blosc2 时使用的 py-blosc2 CMake 配置，然后使用外部 blosc2.h 在 blosc2_ext.c 中重现该失败。完成标准是该配置提供所需的 C99 模式，并且 external-blosc2 构建在没有报告的错误的情况下完成。

由索引模型根据 Issue 内容生成。