$ make
  GEN    doc/manual/manual.is-valid
I/O error : Attempt to load network entity http://docbook.org/xml/5.0/rng/docbook.rng
warning: failed to load external entity "http://docbook.org/xml/5.0/rng/docbook.rng"
Relax-NG parser error : xmlRelaxNGParse: could not load http://docbook.org/xml/5.0/rng/docbook.rng
Relax-NG schema http://docbook.org/xml/5.0/rng/docbook.rng failed to compile
-:1: parser error : Document is empty

^
make: *** [doc/manual/local.mk:32: doc/manual/manual.is-valid] Error 1