fix warning on Cygwin

adk/tools/depmaker.c

@@ -17,6 +17,7 @@
  * along with this program.  If not, see <http://www.gnu.org/licenses/>.
  */
 
+#define _GNU_SOURCE
 #include <ctype.h>
 #include <dirent.h>
 #include <stdlib.h>