doc: specify that xelatex is used as Latex engine

Building the pdf documentation on readthedocs.org fails with pdflatex.

Signed-off-by: Heinrich Schuchardt <xypron.glpk@gmx.de>

doc/conf.py

@@ -31,6 +31,8 @@ from load_config import loadConfig
 # If your documentation needs a minimal Sphinx version, state it here.
 needs_sphinx = '1.3'
 
+latex_engine = 'xelatex'
+
 # Add any Sphinx extension module names here, as strings. They can be
 # extensions coming with Sphinx (named 'sphinx.ext.*') or your custom
 # ones.