GIT version: remake docs

[Hans Aberg]

After first making (& installing) Guile from GIT,
  make pdf
  make clean
  make pdf
failed. It seems 'clean' also removes a file 'standard-library.texi',  
which is needed for 'make pdf'. It is in the guile-1.9.14  
distribution, so it is not a file generated by 'make'.