docs: conf.py: only use CJK if the font is available

If we try to build a book with asian characters with XeLaTeX
and the font is not available, it will produce an error.

So, instead, add a logic at conf.py to detect if the proper
font is installed.

This will avoid an error while building the document, although
the result may not be readable.

Signed-off-by: Mauro Carvalho Chehab <mchehab+samsung@kernel.org>

Documentation/conf.py

@@ -16,6 +16,8 @@ import sys
 import os
 import sphinx
 
+from subprocess import check_output
+
 # Get Sphinx version
 major, minor, patch = sphinx.version_info[:3]
 
@@ -276,13 +278,20 @@ latex_elements = {
         \\setsansfont{DejaVu Sans}
         \\setromanfont{DejaVu Serif}
         \\setmonofont{DejaVu Sans Mono}
+     '''
+}
 
+# At least one book (translations) may have Asian characters
+# with are only displayed if xeCJK is used
+
+cjk_cmd = check_output(['fc-list', '--format="%{family[0]}\n"']).decode('utf-8', 'ignore')
+if cjk_cmd.find("Noto Sans CJK SC") >= 0:
+    print ("enabling CJK for LaTeX builder")
+    latex_elements['preamble']  += '''
 	% This is needed for translations
         \\usepackage{xeCJK}
         \\setCJKmainfont{Noto Sans CJK SC}
-
      '''
-}
 
 # Fix reference escape troubles with Sphinx 1.4.x
 if major == 1 and minor > 3: