x11vnc: fix build without ipv6 support

Fixes http://autobuild.buildroot.net/results/1119dbf055edf281451a9e4216fdebc0b07fffae/

configure script uses --without-ipv6 instead of --disable-ipv6, and configure
option is only used for libraries, not the x11vnc application.

Signed-off-by: Peter Korsgaard <jacmet@sunsite.dk>

package/x11vnc/x11vnc.mk

@@ -11,4 +11,10 @@ X11VNC_CONF_OPT = \
 
 X11VNC_DEPENDENCIES = xlib_libXt xlib_libXext
 
+ifneq ($(BR2_INET_IPV6),y)
+# configure option only used for libvncserver
+X11VNC_CONF_OPT += --without-ipv6
+X11VNC_CONF_ENV += CFLAGS='$(TARGET_CFLAGS) -DX11VNC_IPV6=0'
+endif
+
 $(eval $(autotools-package))