userguide: don't run tex in interactive mode

... as it doesn't behave well with CI builds.

Signed-off-by: Jeremy Kerr <jk@ozlabs.org>
diff --git a/Makefile b/Makefile
index 02ea525..9e5cfa7 100644
--- a/Makefile
+++ b/Makefile
@@ -1,13 +1,14 @@
 
 # we need xelatex for utf-8 support
 tex = xelatex
+tflags = -interaction=nonstopmode -halt-on-error
 
 all: userguide.pdf
 
 .PHONY: all clean
 
 userguide.pdf: userguide/userguide.tex
-	$(tex) $^
+	$(tex) $(tflags) $^
 
 userguide/userguide.tex: rest-api.tex host-management.tex console.tex code-update.tex