Add the '--jobs auto' option to sphinx-build commands

docs/Makefile

@@ -3,7 +3,7 @@
 
 # You can set these variables from the command line, and also
 # from the environment for the first two.
-SPHINXOPTS  ?=
+SPHINXOPTS  ?= --jobs auto
 SPHINXBUILD ?= sphinx-build
 SOURCEDIR    = .
 BUILDDIR     = _build

docs/devel/Makefile

-SPHINXOPTS ?= -t devel_doc
+SPHINXOPTS ?= -t devel_doc --jobs auto
 SPHINXOPTCOLOR ?= --color
 SPHINXBUILD = sphinx-build
 SOURCEDIR = .