[PATCH 07/13] manual: Fix missing declaration in strdupa example.

[Collin Funk]

Without _GNU_SOURCE defined this file fails to compile with the
following error:

    $ gcc manual/examples/strdupa.c
    manual/examples/strdupa.c: In function ‘main’:
    manual/examples/strdupa.c:27:19: error: implicit declaration of function ‘strdupa’; did you mean ‘strdup’? [-Wimplicit-function-declaration]
       27 |   char *wr_path = strdupa (path);
          |                   ^~~~~~~
          |                   strdup
    manual/examples/strdupa.c:27:19: error: initialization of ‘char *’ from ‘int’ makes pointer from integer without a cast [-Wint-conversion]
---
 manual/examples/strdupa.c | 1 +
 1 file changed, 1 insertion(+)

diff --git a/manual/examples/strdupa.c b/manual/examples/strdupa.c
index 7fd2293d9f..004a3b3d7c 100644
--- a/manual/examples/strdupa.c
+++ b/manual/examples/strdupa.c
@@ -15,6 +15,7 @@
    along with this program; if not, see <https://www.gnu.org/licenses/>.
 */
 
+#define _GNU_SOURCE 1
 #include <paths.h>
 #include <string.h>
 #include <stdio.h>
-- 
2.51.0