There seems to be no end of bugs in xmlto-related toolchains on the various platforms. Let's investigate either (a) committing built documentation to the source tree so processing is not attempted during build time, (b) switching away from xml to some portable vanilla like tex, or (c) something else.
*** Bug 18776 has been marked as a duplicate of this bug. ***
Fixed in commit 1ed33130e763e58d39af2965 and d9c423577b3c09fdbf01ea. Documents are now stored in the source tree and must now be updated manually using scripts/generate-docs.