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>