Set Sphinx to use xetex

xetex prevents a unicode error seen in pdflatex

Signed-off-by: Richard Berg <rberg@bitwise.io>

docs/Makefile

@@ -160,7 +160,7 @@ latexpdf: templates cli
 	@mkdir -p source/_static
 	$(SPHINXBUILD) -b latex $(ALLSPHINXOPTS) $(BUILDDIR)/latex
 	@echo "Running LaTeX files through pdflatex..."
-	$(MAKE) -C $(BUILDDIR)/latex all-pdf
+	$(MAKE) PDFLATEX=xelatex -C $(BUILDDIR)/latex all-pdf
 	@echo "pdflatex finished; the PDF files are in $(BUILDDIR)/latex."
 
 latexpdfja: templates cli