IIOD: Remove possibility to run with the XML backend

It was used for test purposes only, at the infancy of the daemon.
Removing this feature allows IIOD to run with versions of the libiio
library that don't provide the XML backend.

Signed-off-by: Paul Cercueil <paul.cercueil@analog.com>
Makefile: Conditionally build the backends

It is now possible to de-select the local backend by setting the
environment variable WITH_LOCAL_BACKEND=no, and disable the network
backend (and by extension the XML backend) by setting the variable
WITH_NETWORK_BACKEND=no.

Signed-off-by: Paul Cercueil <paul.cercueil@analog.com>
examples/Makefile: Fix build issues when libiio is not yet installed

Signed-off-by: Paul Cercueil <paul.cercueil@analog.com>