Fix warning about redefinition of _GNU_SOURCE

G++ implicitly defines _GNU_SOURCE=1 so define it consistently to avoid
a warning about redefinition.

lib/SCSI2SD/src/scsi2sd-util6/libzipper-1.0.4/port/strerror_gnu.cc

@@ -15,7 +15,7 @@
 //	You should have received a copy of the GNU General Public License
 //	along with libzipper.  If not, see <http://www.gnu.org/licenses/>.
 
-#define _GNU_SOURCE
+#define _GNU_SOURCE 1
 
 #include "../strerror.hh"