Use nproc + 1

Doc/Makefile

@@ -217,7 +217,7 @@ dist:
 	# as otherwise the full latexmk process is run twice.
 	# ($$ is needed to escape the $; https://www.gnu.org/software/make/manual/make.html#Basics-of-Variable-References)
 	-sed -i 's/: all-$$(FMT)/:/' build/latex/Makefile
-	(cd build/latex; $(MAKE) clean && $(MAKE) --jobs=12 --output-sync LATEXMKOPTS='-quiet' all-pdf && $(MAKE) FMT=pdf zip bz2)
+	(cd build/latex; $(MAKE) clean && $(MAKE) --jobs=$((`nproc`+1)) --output-sync LATEXMKOPTS='-quiet' all-pdf && $(MAKE) FMT=pdf zip bz2)
 	cp build/latex/docs-pdf.zip dist/python-$(DISTVERSION)-docs-pdf-a4.zip
 	cp build/latex/docs-pdf.tar.bz2 dist/python-$(DISTVERSION)-docs-pdf-a4.tar.bz2
 	@echo "Build finished and archived!"
@@ -227,7 +227,7 @@ dist:
 	rm -rf build/latex
 	$(MAKE) latex PAPER=letter
 	-sed -i 's/: all-$$(FMT)/:/' build/latex/Makefile
-	(cd build/latex; $(MAKE) clean && $(MAKE) --jobs=12 --output-sync LATEXMKOPTS='-quiet' all-pdf && $(MAKE) FMT=pdf zip bz2)
+	(cd build/latex; $(MAKE) clean && $(MAKE) --jobs=$((`nproc`+1)) --output-sync LATEXMKOPTS='-quiet' all-pdf && $(MAKE) FMT=pdf zip bz2)
 	cp build/latex/docs-pdf.zip dist/python-$(DISTVERSION)-docs-pdf-letter.zip
 	cp build/latex/docs-pdf.tar.bz2 dist/python-$(DISTVERSION)-docs-pdf-letter.tar.bz2
 	@echo "Build finished and archived!"