This should silence some warnings about the jobserver, but also make
it easier to build the docs where GNU make is called something other
than make.
Based on a patch from aidecoe.
$(SPHINXBUILD) -b texinfo $(ALLSPHINXOPTS) $(DOCBUILDDIR)/texinfo
sphinx-info: sphinx-texinfo
- make -C $(DOCBUILDDIR)/texinfo info
+ $(MAKE) -C $(DOCBUILDDIR)/texinfo info
# Use the man page converter that is available. We should never depend
# on MAN_ROFF_FILES if a converter is not available.