MAI: update script make-pdf

[johnyf]

• use the Python script list_files.py to list
  the paths of Markdown files to typeset,
  because one of the headers in the table of contents
  src/toc.md contains a /, resulting in
  an erroneous path passed to pandoc

• pass to pandoc the option --latex-engine=xelatex
  because one of the source files contains a UTF8 character

• use pdftk, which is faster than convert

• fix typos in echo and printf messages