NixOS/nix — pull request #413 by darealshinji (2014-12-10 23:11:53+00:00)
Title: allow custom docbook paths/URLs, use a working URL for docbook.rng

Useful for offline builds:
```
docbookrng=/usr/share/xml/docbook/schema/rng/5.0/docbook.rng \
docbookxsl=/usr/share/xml/docbook/stylesheet/docbook-xsl \
make
```