Bug#685114: freefoam: please build docs only as needed

[Aaron M. Ucko]

Source: freefoam
Version: 0.1.0+dfsg-1
Severity: serious
Justification: fails to build from source

Builds of freefoam on armel and armhf have been failing with errors
building doc/UserGuide/images/tut_ico_cavity_block.pdf:

  cd /.../freefoam-0.1.0+dfsg/obj-arm-linux-gnueabi/doc/UserGuide/CMakeFiles/tut_ico_cavity_block_pdf.dir && /usr/bin/asy -fpdf -tex pdflatex tut_ico_cavity_block
  /usr/share/asymptote/plain_Label.asy: 633.5: runtime: ! Extra }, or forgotten $.
  <*> \setbox\ASYbox=\hbox{$33}
                               
  
  *! Missing $ inserted.
  <inserted text> 
                  $
  <*> 
      
  (Please type a command or say `\end')
  *(Please type a command or say `\end')
  *
  make[3]: *** [doc/UserGuide/images/tut_ico_cavity_block.pdf] Error 1

AFAICT, these errors are spurious, presumably stemming from a bug in
asymptote or perhaps pdflatex; however, you could avoid them
altogether, and allow for more efficient builds, by arranging to build
documentation only when actually building the architecture-independent
-doc packages.

Could you please look into doing so (and moving the relevant build
dependencies into Build-Depends-Indep so that builds covering only
architecture-dependent packages can know to do without them)?

Thanks!