New review comment by sgn on void-packages repository

https://github.com/void-linux/void-packages/pull/39154#discussion_r965524818

Comment:
We may need to:
```cpp
#ifndef _POSIX_C_SOURCE
#define _POSIX_C_SOURCE 200112L
#endif
```

Just in case the `boost/align/detail/aligned_alloc_posix.hpp` needs something only available when that macro is defined?