Build fails: RFC related warnings followed by errors

[iggysha]

Warnings:

WARNING: Unable to load the include file at https://xml2rfc.ietf.org/public/rfc/bibxml/reference.RFC.2360.xml
WARNING: Unable to load the include file at https://xml2rfc.ietf.org/public/rfc/bibxml/reference.RFC.8200.xml
...
...

And then errors:

ERROR: Unable to validate the XML document: draft-rquattle-spinel-core.xml
: Line 3375: Element references content does not follow the DTD, expecting (reference)+, got ()
: Line 3379: Element references content does not follow the DTD, expecting (reference)+, got ()
: Line 3789: Syntax of value for attribute target of xref is not valid
: Line 92: IDREF attribute target references an unknown ID "RFC2360"
: Line 44: IDREF attribute target references an unknown ID "RFC8200"
...
...