make: Silence some xelatex output

Currently build logs from GitLab CI around around 7 megabytes each. Of this,
around 2 megabytes is latex output. This is quite silly as essentially all of
this output is unresolved references in the early latex iterations.
Here we silence this output.

However, to make sure that we don't silence errors we allow each xelatex
invocation besides the last to fail.

rules/sphinx.mk

@@ -56,13 +56,18 @@ pdf_$1 : $1/$2.pdf
 pdf : pdf_$1
 
 ifneq "$$(BINDIST)" "YES"
+# N.B. If we don't redirect latex output to /dev/null then we end up with literally
+# 30% of the build output being warnings, even in a successful build. However,
+# to make sure that we don't silence errors we allow each xelatex invocation
+# besides the last to fail.
+
 $1/$2.pdf: $1/conf.py $$($1_RST_SOURCES)
 	$(SPHINXBUILD) -b latex -d $1/.doctrees-pdf $(SPHINXOPTS) $1 $1/build-pdf/$2
-	cd $1/build-pdf/$2 ; xelatex -halt-on-error $2.tex
-	cd $1/build-pdf/$2 ; xelatex -halt-on-error $2.tex
-	cd $1/build-pdf/$2 ; xelatex -halt-on-error $2.tex
+	cd $1/build-pdf/$2 ; xelatex -halt-on-error $2.tex 2>/dev/null >/dev/null || true
+	cd $1/build-pdf/$2 ; xelatex -halt-on-error $2.tex 2>/dev/null >/dev/null || true
+	cd $1/build-pdf/$2 ; xelatex -halt-on-error $2.tex 2>/dev/null >/dev/null || true
 	cd $1/build-pdf/$2 ; makeindex $2.idx
-	cd $1/build-pdf/$2 ; xelatex -halt-on-error $2.tex
+	cd $1/build-pdf/$2 ; xelatex -halt-on-error $2.tex 2>/dev/null >/dev/null || true
 	cd $1/build-pdf/$2 ; xelatex -halt-on-error $2.tex
 	cp $1/build-pdf/$2/$2.pdf $1/$2.pdf
 endif