lib: cpp: Enable POSIX and GNU C extensions for libc++

LLVM C++ Standard Library aka. libc++ makes use of POSIX and GNU C
extensions in its headers.

While far from ideal, there is no other option but to globally enable these
extensions for C++ source files in order to ensure that libc++ headers can
be used.

Signed-off-by: Stephanos Ioannidis <root@stephanos.io>

Author: stephanosio

lib/cpp/CMakeLists.txt

@@ -1,5 +1,13 @@
 # SPDX-License-Identifier: Apache-2.0
 
+if(CONFIG_LIBCXX_LIBCPP)
+  # LLVM C++ Standard Library makes use of POSIX and GNU C extension functions
+  # in its headers.
+  zephyr_compile_definitions($<$<COMPILE_LANGUAGE:CXX>:_POSIX_C_SOURCE=200809L>)
+  zephyr_compile_definitions($<$<COMPILE_LANGUAGE:CXX>:_XOPEN_SOURCE=700>)
+  zephyr_compile_definitions($<$<COMPILE_LANGUAGE:CXX>:_GNU_SOURCE>)
+endif()
+
 add_subdirectory(abi)
 
 add_subdirectory_ifdef(CONFIG_MINIMAL_LIBCPP minimal)