title: bookdown Error 'CTeX fontset `mac' is unavailable in current mode.' when using output format pdf_book & pdf_document2
body: <p>I'm using bookdown with Rstudio + tinyTex + pandoc to write a report in Chinese. Because I need to use cross reference, I have to set the output format to pdf_document2, but I got an issue about CTeX fontest. </p>
  
  <p>First, here is the YAML part:</p>
  
  <pre><code>---
  title: "title"
  author:
    - author
  documentclass: ctexart
  site: bookdown::bookdown_site
  output:
    bookdown::pdf_document2
  bibliography: bib_v1.bib
  csl: chinese-author-date.csl
  link-citations: yes
  colorlinks: yes
  classoption: "hyperref,"
  ---
  </code></pre>
  
  <p>And there's some wrong when build the book, which says 'CTeX fontset `mac' is unavailable in current mode.', and here's the whole error info:</p>
  
  <pre><code>tlmgr search --file --global '/mac'
  TeX Live 2017 is frozen forever and will no
  longer be updated.  This happens in preparation for a new release.
  
  If you are interested in helping to pretest the new release (when
  pretests are available), please read     http://tug.org/texlive/pretest.html.
  Otherwise, just wait, and the new release will be ready in due time.
  ! Critical ctex error: "fontset-unavailable"
  ! 
  ! CTeX fontset 'mac' is unavailable in current mode.
  ! 
  ! See the ctex documentation for further information.
  ! 
  ! For immediate help type H &lt;return&gt;.
  !...............................................  
  
  ! 
  ! CTeX fontset 'mac' is unavailable in current mode.
  ! 
  ! See the ctex documentation for further information.
  ! 
  ! For immediate help type H &lt;return&gt;.
  !...............................................  
  
  ! CTeX fontset 'mac' is unavailable in current mode.
  ! 
  ! See the ctex documentation for further information.
  ! 
  ! For immediate help type H &lt;return&gt;.
  !...............................................  
  
  ! 
  ! See the ctex documentation for further information.
  ! 
  ! For immediate help type H &lt;return&gt;.
  !...............................................  
  
  ! See the ctex documentation for further information.
  ! 
  ! For immediate help type H &lt;return&gt;.
  !...............................................  
  
  ! 
  ! For immediate help type H &lt;return&gt;.
  !...............................................  
  
  ! For immediate help type H &lt;return&gt;.
  !...............................................  
  
  Error: Failed to compile _main.tex. See _main.log for more info.
  In addition: Warning message:
  In parse_packages(logfile, quiet = c(TRUE, FALSE, FALSE)) :
    Failed to find a package that contains mac
  Please delete _main.Rmd after you finish debugging the error.
  Execution halted
  
  Exited with status 1.
  </code></pre>
  
  <p>I'm not familiar with LaTeX, and don't know how to fix the issue. I've tried to build the <a href="https://github.com/yihui/bookdown-chinese" rel="nofollow noreferrer">bookdown-Chinese demo</a> here, and it succeed. I also tried to use the template of this demo in my own file, or use the output format pdf_book, but I still got the same problem.</p>
  
  <p>So is there something I can do to fix it?</p>
  
  <p>Thanks very much for your help!</p>