posix: manually define _POSIX_C_SOURCE in lib/posix

This PR adds declarations for setenv and unsetenv, which require
the application conformance feature test macro _POSIX_C_SOURCE
to be defined to a value greater than or equal to 200112L.

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>

lib/posix/shell/CMakeLists.txt

@@ -1,6 +1,12 @@
 # Copyright (c) 2024 Meta
 # SPDX-License-Identifier: Apache-2.0
 
+zephyr_library()
+# For setenv() and unsetenv()
+zephyr_library_compile_options(-U_POSIX_C_SOURCE -D_POSIX_C_SOURCE=200809L)
+# For getenv_r() visibility
+zephyr_library_compile_definitions(_BSD_SOURCE)
+
 zephyr_library_sources_ifdef(CONFIG_POSIX_SHELL posix_shell.c)
 zephyr_library_sources_ifdef(CONFIG_POSIX_UNAME_SHELL uname.c)
 zephyr_library_sources_ifdef(CONFIG_POSIX_ENV_SHELL env.c)