Avoid using `#include_next` GNU extension ... 

* `#include_next` is non-POSIX (a GNU extension) and is currently used in:
  * `include/bsd_stdlib.h`
  * `include/bsd_string.h`
  * `include/sys/types.h`
  * `include/sys/time.h`
  * `include/sys/stat.h`
  * `include/bsd_termios.h`
  * `include/bsd_fcntl.h`
  * `include/bsd_unistd.h`
  * `include/libgen.h`
  * `include/poll.h`
