When building the target doc/manual/manual.is-valid,

xsltproc is passed the option --nonet. Since the document which is fetched for this target is http://docbook.org/xml/5.0/rng/docbook.rng,

xsltproc fails with the following output:

GEN Makefile.config
GEN doc/manual/manual.xmli
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

^
-:1: parser error : Start tag expected, '<' not found

^
doc/manual/local.mk:29: recipe for target 'doc/manual/manual.is-valid' failed
make: *** [doc/manual/manual.is-valid] Error 1

Removing --nonet from the xsltproc lines fixes this error.