example/x11: Build only when sys/un.h is found by configure

The example can't be built on systems without AF_UNIX sockets.

configure.ac

@@ -220,7 +220,8 @@ AC_HELP_STRING([--disable-hidden-symbols],[Leave all symbols with default visibi
 AC_CHECK_HEADERS([errno.h fcntl.h stdio.h stdlib.h unistd.h sys/uio.h])
 AC_CHECK_HEADERS([sys/select.h sys/socket.h sys/ioctl.h sys/time.h])
 AC_CHECK_HEADERS([arpa/inet.h netinet/in.h])
-AC_CHECK_HEADERS([sys/un.h])
+AC_CHECK_HEADERS([sys/un.h], [have_sys_un_h=yes], [have_sys_un_h=no])
+AM_CONDITIONAL([HAVE_SYS_UN_H], test "x$have_sys_un_h" = xyes)
 
 case $host in
   *-*-cygwin* | *-*-cegcc*)

example/Makefile.am

@@ -7,7 +7,11 @@ noinst_PROGRAMS = direct_tcpip ssh2 scp scp_nonblock scp_write		\
  scp_write_nonblock sftp sftp_nonblock sftp_write sftp_write_nonblock	\
  sftp_mkdir sftp_mkdir_nonblock sftp_RW_nonblock sftp_write_sliding	\
  sftpdir sftpdir_nonblock ssh2_exec ssh2_agent ssh2_echo sftp_append	\
- subsystem_netconf tcpip-forward x11
+ subsystem_netconf tcpip-forward
+
+if HAVE_SYS_UN_H
+noinst_PROGRAMS += x11
+endif
 
 AM_CPPFLAGS = -I$(top_srcdir)/include -I$(top_builddir)/example
 LDADD = $(top_builddir)/src/libssh2.la