stable docs build failure

[Graham Percival]

I'd like to backport the doc changes from the past few months, but I 
can't build the stable docs.  (before changing anything, I mean!)


The build fails on this snippet, lily-910891298.ly on my system:

% ****************************************************************
% ly snippet:
% ****************************************************************
\score {
  \transpose c c' {
    \set Score.timing = ##f
    \set Score.defaultBarType = "empty"
    \override NoteHead #'style = #'neomensural
    \override Staff.TimeSignature #'style = #'neomensural
    \clef "petrucci-g"
    \[ c'\maxima g \]
    s4
    \[ d\longa c\breve f e d \]
    s4
    \[ c'\maxima d'\longa \]
    s4
    \[ e'1 a g\breve \]
  }
  \layout {
    \context {
      \Voice
      \remove Ligature_bracket_engraver
      \consists Mensural_ligature_engraver
    }
  }
}



Error messages:

Calculating line breaks...
programming error: Note_spacing::get_spacing(): Common refpoint incorrect
continuing, cross fingers
programming error: Note_spacing::get_spacing(): Common refpoint incorrect
continuing, cross fingers
...
Layout output to 
`lily-910891298-1.eps'.../home/gperciva/usr/src/stable-lilypond/lilypond/out/share/lilypond/current/scm/framework-ps.scm:487:24: 
In procedure inexact->exact in expression (inexact->exact (round #)):
/home/gperciva/usr/src/stable-lilypond/lilypond/out/share/lilypond/current/scm/framework-ps.scm:487:24: 
Numerical overflow
...