FreeBSD needs libutil.h for forkpty()

The header is not available everywhere, so only include it if it
is present.  (The extra include should be harmless on Linux by
inspection.)

configure.ac

@@ -103,7 +103,7 @@ AS_IF([test x"$with_skalibs" != xno],
 # Checks for header files.
 AC_CHECK_HEADERS([arpa/inet.h curses.h fcntl.h langinfo.h limits.h locale.h netinet/in.h stddef.h stdint.h stdlib.h string.h sys/ioctl.h sys/resource.h sys/socket.h sys/time.h term.h termios.h unistd.h wchar.h wctype.h], [], [AC_MSG_ERROR([Missing required header file.])])
 
-AC_CHECK_HEADERS([pty.h util.h])
+AC_CHECK_HEADERS([pty.h util.h libutil.h])
 AC_CHECK_HEADERS([endian.h sys/endian.h])
 
 # Checks for typedefs, structures, and compiler characteristics.

src/frontend/mosh-server.cc

@@ -20,6 +20,9 @@
 
 #include <errno.h>
 #include <locale.h>
+#ifdef HAVE_LIBUTIL_H
+#include <libutil.h>
+#endif
 #include <string.h>
 #include <termios.h>
 #include <unistd.h>