Fix config.h to avoid conflicting _POSIX_C_SOURCE

[Lapshin]

When _DEFAULT_SOURCE is defined, it is unnecessary to define _POSIX_C_SOURCE explicitly, as libc implicitly sets it (typically to 200809L). Explicitly defining _POSIX_C_SOURCE can lead to compatibility issues on systems where libc does not default to 200809L

This change avoids redundant or conflicting macro definitions for better portability.

You can read more details here https://man7.org/linux/man-pages/man7/feature_test_macros.7.html