xcmd: define _GNU_SOURCE

This commit may solve #143.
If _GNU_SOURCE is defined, WEXITSTATUS defined in stdlib.h is
visible.

Signed-off-by: Masatake YAMATO <yamato@redhat.com>
Merge pull request #158 from masatake/use-WEXITSTATUS-in-stdlib-h-3

xcmd: define _GNU_SOURCE