x11vnc: use external libvncserver

By default x11vnc compiles its dependency libvncserver from sources
included in its own source tree. We override this behaviour to use
the externally provided libvncserver from a separate recipe.

Signed-off-by: Ioan-Adrian Ratiu <adrian.ratiu@ni.com>
Signed-off-by: Martin Jansa <Martin.Jansa@gmail.com>