Creating the manual should work out of the box, if you install the freshly released SCons v2.3.1. You might have to uncomment the PDF creation commands if you don't have a PDF renderer like fop or xep installed.
But for simple HTML output you now only need SCons and one of the libxml2 or lxml Python bindings.
The Qt4 Tool has precompiled manuals at http://www.scons.org/wiki/Qt4Tool , and it's very similar to Qt5.
But please try to compile the manual yourself and report any errors you find.
What do you mean when you say "you're stuck"? Can't even download the "local" version of SCons (doesn't require you to install it systemwide) and start it?
SCons v.2.3.0 doesn't have the "docbook" Tool as core component, it was only added for v2.3.1.
So the other option would be to download it separately from its repo (as given in the ToolsIndex at the Wiki).
If you don't have the Docbook stylesheets installed in your system, you'll also want to remove the "DOCBOOK_XSL=..." parameter in the SConstruct.
If you should still have trouble getting the DocBook Tool to work, I've added a README.rst to the repo a few seconds ago.
So you can simply go to the frontpage of scons_qt5 and read the manual there now.
The error message says that in l. 35 of your SConstruct "conf.CheckCXX()" gets called. Did you add it? I'm asking because I searched the full source tree for the Qt5 tool and SCons v2.3.1 itself, and found no match.
Sorry for asking this, but have you already successfully run some other build with SCons, or are these your very first steps?
In the latter case you might want to write to our mailing list at email@example.com, where you can get more and faster help because more people listen.
If it's okay with you like that, sure. I didn't want to cut off this conversation, but simply wondered what your level of knowledge with SCons would be.
From what I could see so far, I'd still suspect that (for some unknown reason) the DocBook tool is not able to find the libxml2/lxml bindings, and also (this is the last fallback usually) can't detect a "xsltproc" executable on your current PATH.
That's where the funny name "o" for the executable comes from, that SCons is trying to call.
Anyway, we'll leave it at that for now. If you should experience further problems and have more questions, please feel invited to open a new issue report at the offending tool, or come over to the SCons user mailing list.