s4:heimdal: fix build on FreeBSD

Patch from Timur I. Bakeyev sent to samba-technical:

Heimdal requires openpty() presence. FreeBSD has in in standard libc, so
autodetection works, but compilation fails, as declaration of this function is
missing.

This patch adds proper header detection and inclusion for openpty().

source4/heimdal/lib/roken/rkpty.c

@@ -48,6 +48,9 @@
 #ifdef HAVE_UTIL_H
 #include <util.h>
 #endif
+#ifdef HAVE_LIBUTIL_H
+#include <libutil.h>
+#endif
 
 #include "roken.h"
 #include <getarg.h>

source4/heimdal_build/internal.m4

@@ -52,7 +52,6 @@ AC_CHECK_HEADERS([				\
 	errno.h					\
 	inttypes.h				\
 	netdb.h					\
-	pty.h					\
 	signal.h				\
 	sys/bswap.h				\
 	sys/file.h				\
@@ -164,6 +163,7 @@ m4_include(heimdal/cf/find-func-no-libs.m4)
 m4_include(heimdal/cf/find-func-no-libs2.m4)
 m4_include(heimdal/cf/resolv.m4)
 
+AC_CHECK_HEADERS([pty.h util.h libutil.h])
 
 AC_CHECK_LIB_EXT(util, OPENPTY_LIBS, openpty)