the sphinxcontrib.image extension was broken with sphinx 16.x.
however, sphinx 15.x breaks with newer version of the multiprocessor module.
so we suspend the thumbnail processing and lift the lock to sphinx 15.x
also, the number of parallel sphinx tasks is can be overridden with SPHINXEXTRA="-j #'.
default is to try use all local CPU cores.
Uses the sphinxcontrib-spelling extension and requires PyEnchant and
aspell-en to be installed.
Adds the optional make target 'spelling' which produces the file
spelling/output.txt. It contains all words it detects.
Due to the large number of false positives, words in
utils/sphinx-config/false_positives.txt are ignored.
