Recurse into subdirectory x11vnc/ when configuring with --with-x11vnc

Since we separated the packages LibVNCServer and x11vnc, there is
a configure switch --with-x11vnc, without which x11vnc is not built.

However, even _with_ this switch, it is not built, because the Makefile
would not recurse into the x11vnc/ subdirectory.  Fix that.
Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>

Makefile.am

 CFLAGS=-g -Wall
 
-SUBDIRS=libvncserver examples contrib libvncclient vncterm classes client_examples test
+if WITH_X11VNC
+X11VNC=x11vnc
+endif
+
+SUBDIRS=libvncserver examples contrib libvncclient vncterm classes client_examples test $(X11VNC)
 DIST_SUBDIRS=libvncserver examples contrib libvncclient vncterm classes client_examples test
 
 bin_SCRIPTS = libvncserver-config

configure.ac

@@ -704,6 +704,7 @@ for directory in packages OpenLinux redhat RedHat rpm RPM "" ; do
 done
 echo "$RPMSOURCEDIR"
 AM_CONDITIONAL(HAVE_RPM, test "$RPMSOURCEDIR" != "NOT-FOUND")
+AM_CONDITIONAL(WITH_X11VNC,  test "$build_x11vnc" = "yes")
 AC_SUBST(RPMSOURCEDIR)
 
 AC_CONFIG_FILES([Makefile