See Building The Documentation :
J.3.2. Manpages
We use the DocBook XSL stylesheets to convert DocBook refentry pages to *roff output suitable for man pages. The man pages are also distributed as a tar archive, similar to the HTML version. To create the man pages, use the commands:
cd doc/src/sgml
gmake man
This is part of the global make world
but starting from the subdirectory you may see what specifically fails, looking at of the whole output rather than just the end.
From the piece of output you pasted, it seems it was trying to build the general documentation, not a manpage, and it produces it on screen rather that into a file, which is weird, or it's a very long error message but the interesting part would be above.
In any case, make sure you have a properly working xsltproc
in your system.