Define _GNU_SOURCE to work around pcap/musl bug

musl libc seems to not define things like `u_short` unless
`_GNU_SOURCE` or `_BSD_SOURCE` is defined. And pcap header files
assume those typedefs exist.

This should probably be fixed by pcap, but this commit is a
workaround.

Fixes #55

src/arping.c

@@ -31,6 +31,7 @@
  *  with this program; if not, write to the Free Software Foundation, Inc.,
  *  51 Franklin Street, Fifth Floor, Boston, MA 02110-1301 USA.
  */
+#define _GNU_SOURCE
 #if HAVE_CONFIG_H
 #include "config.h"
 #endif

src/arping_test.c

@@ -16,8 +16,8 @@
  *  with this program; if not, write to the Free Software Foundation, Inc.,
  *  51 Franklin Street, Fifth Floor, Boston, MA 02110-1301 USA.
  */
-#include"config.h"
 #define _GNU_SOURCE
+#include"config.h"
 #include<assert.h>
 #include<errno.h>
 #include<fcntl.h>

src/findif_getifaddrs.c

@@ -19,6 +19,7 @@
 /**
  * Most modern systems should have getifaddrs().
  */
+#define _GNU_SOURCE
 #if HAVE_CONFIG_H
 #include "config.h"
 #endif

src/fuzz_pingip.c

@@ -16,6 +16,8 @@
  *  with this program; if not, write to the Free Software Foundation, Inc.,
  *  51 Franklin Street, Fifth Floor, Boston, MA 02110-1301 USA.
  */
+#define _GNU_SOURCE
+
 #include<arpa/inet.h>
 #include<errno.h>
 #include<inttypes.h>

src/fuzz_pingmac.c

@@ -1,3 +1,5 @@
+#define _GNU_SOURCE
+
 #include<errno.h>
 #include<string.h>
 #include<stdio.h>

src/mock_libpcap.c

@@ -1,3 +1,5 @@
+#define _GNU_SOURCE
+
 #include<pcap.h>
 
 #define UNUSED(x) (void)(x)

src/unix.c

@@ -16,6 +16,7 @@
  *  with this program; if not, write to the Free Software Foundation, Inc.,
  *  51 Franklin Street, Fifth Floor, Boston, MA 02110-1301 USA.
  */
+#define _GNU_SOURCE
 #if HAVE_CONFIG_H
 #include "config.h"
 #endif