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 <[email protected]>

Author: cfriedt

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)