* pex-djgpp.c: Include string.h, fcntl.h, unistd.h, and
sys/stat.h.
(pex_init): Fix last argument to pex_init_common.
(pex_djgpp_exec_child): Remove leading underscore from _open,
_dup, _dup2, _close, and _spawnv/_spawnvp. Replace `program',
which is undeclared, with `executable', which was unused. Remove
unused variable `e'. Fix casting of last arg to spawnv/spawnvp.
(pex_djgpp_wait): Declare arguments with ATTRIBUTE_UNUSED.