More for mdb_env_copy().

Fix #define _GNU_SOURCE = 1 to match any command line -D_GNU_SOURCE.
.gitignore += mdb_copy.

libraries/libmdb/.gitignore

@@ -1,6 +1,7 @@
 mtest
 mtest[23456]
 testdb
+mdb_copy
 mdb_stat
 *.[ao]
 *.so

libraries/libmdb/mdb.c

@@ -32,7 +32,7 @@
  * ACTION OF CONTRACT, NEGLIGENCE OR OTHER TORTIOUS ACTION, ARISING OUT OF
  * OR IN CONNECTION WITH THE USE OR PERFORMANCE OF THIS SOFTWARE.
  */
-#define _GNU_SOURCE
+#define _GNU_SOURCE 1
 #include <sys/types.h>
 #include <sys/stat.h>
 #include <sys/param.h>