Fix build when libutil.h exists but is deprecated

configure.ac

@@ -159,6 +159,11 @@ AC_CHECK_DECL([signalfd],
      [Define if signalfd is available.])],
   , [[#include <sys/signalfd.h>]])
 
+AC_CHECK_DECL([forkpty],
+  [AC_DEFINE([FORKPTY_IN_LIBUTIL], [1],
+     [Define if libutil.h necessary for forkpty().])],
+  , [[#include <libutil.h>]])
+
 AC_CHECK_DECL([htobe64],
   [AC_DEFINE([HAVE_HTOBE64], [1],
   [Define if htobe64 and friends exist.])],

src/frontend/mosh-server.cc

@@ -20,9 +20,6 @@
 
 #include <errno.h>
 #include <locale.h>
-#ifdef HAVE_LIBUTIL_H
-#include <libutil.h>
-#endif
 #include <string.h>
 #include <termios.h>
 #include <unistd.h>
@@ -59,6 +56,10 @@ extern "C" {
 #include <util.h>
 #endif
 
+#if FORKPTY_IN_LIBUTIL
+#include <libutil.h>
+#endif
+
 #include "networktransport.cc"
 
 typedef Network::Transport< Terminal::Complete, Network::UserStream > ServerConnection;