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>