Support luatex for building documentation

• doc: Enable luatex for generating PDF documentation

  Contrary to XeTeX, luatex supports font expansion. This feature is used for the new microtype extensions in texinfo.tex.

  You can select luatex by setting the environment variables

  PDFTEX=luatex
  PDFLATEX=lualatex

  for compilation.

• build: Use double quotes if passing environment variables like $TEX

  Users might override these variables with strings that contain spaces, for example

  PDFTEX="luatex --fmt=luatex-plain"