Added parallelism support via --jobs to docs build Makefile rule.

docs/Makefile

@@ -8,6 +8,7 @@ SPHINXBUILD   ?= sphinx-build
 PAPER         ?=
 BUILDDIR      ?= _build
 LANGUAGE      ?=
+JOBS          ?= auto
 
 # Set the default language.
 ifndef LANGUAGE
@@ -21,7 +22,7 @@ LANGUAGEOPT = $(firstword $(subst _, ,$(LANGUAGE)))
 # Internal variables.
 PAPEROPT_a4     = -D latex_paper_size=a4
 PAPEROPT_letter = -D latex_paper_size=letter
-ALLSPHINXOPTS   = -n -d $(BUILDDIR)/doctrees -D language=$(LANGUAGEOPT) $(PAPEROPT_$(PAPER)) $(SPHINXOPTS) .
+ALLSPHINXOPTS   = -n -d $(BUILDDIR)/doctrees -D language=$(LANGUAGEOPT) --jobs $(JOBS) $(PAPEROPT_$(PAPER)) $(SPHINXOPTS) .
 # the i18n builder cannot share the environment and doctrees with the others
 I18NSPHINXOPTS  = $(PAPEROPT_$(PAPER)) $(SPHINXOPTS) .