libretro-common: Fix redefinition warnings of _POSIX_C_SOURCE (#15748)

Small fix to only define `_POSIX_C_SOURCE` if it's not already defined.

```
rthreads/rthreads.c:25: warning: "_POSIX_C_SOURCE" redefined
```
This commit is contained in:
Rob Loach 2024-05-16 16:05:58 -04:00 committed by GitHub
parent ec8358c783
commit 395f925941
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@ -22,9 +22,11 @@
#ifdef __unix__
#ifndef __sun__
#ifndef _POSIX_C_SOURCE
#define _POSIX_C_SOURCE 199309
#endif
#endif
#endif
#include <stdlib.h>
#include <string.h>