Fix compilation on FreeBSD 8.2

FreeBSD has a recent version of autotools which induces some issues.
This patch fixes it by improving headers detection code in configure.in.

configure.in

@@ -90,7 +90,13 @@ AC_INIT(configure.in)
 # Checks for libraries.
 
 # Checks for header files.
-    AC_CHECK_HEADERS([arpa/inet.h inttypes.h limits.h net/if.h netdb.h netinet/in.h poll.h signal.h stdint.h stdlib.h string.h syslog.h sys/ioctl.h sys/prctl.h sys/socket.h sys/syscall.h sys/time.h unistd.h windows.h winsock2.h ws2tcpip.h])
+    AC_CHECK_HEADERS([arpa/inet.h inttypes.h limits.h netdb.h poll.h signal.h stdint.h stdlib.h string.h syslog.h sys/ioctl.h sys/prctl.h sys/socket.h sys/syscall.h netinet/in.h sys/time.h unistd.h windows.h winsock2.h ws2tcpip.h])
+   AC_CHECK_HEADERS([sys/socket.h net/if.h], [], [],
+    [[#ifdef HAVE_SYS_SOCKET_H
+#include <sys/socket.h>
+#endif
+    ]])
+
 
 # Checks for typedefs, structures, and compiler characteristics.
     AC_C_INLINE