posix: manually define _POSIX_C_SOURCE in lib/posix

This PR adds declarations for the application conformance
feature test macro _POSIX_C_SOURCE.

It needs to be defined to value greater than or equal to 200112L
by the appplication.

However, Zephyr currently does not have a simple and consistent
means of specifying this value for POSIX samples, tests,
applications, and other libraries.

Signed-off-by: Christopher Friedt <cfriedt@meta.com>