This replaces the old hack which was using a local Linux-specific symlink. Now,
the Dynare default is the same as in Sphinx, i.e. rely on a CDN for delivering
MathJax.
Adapt our CI accordingly to continue using MathJax from the Debian package
there (to avoid privacy issues related to the use of a CDN).
Mostly working. The testsuite remains to be done.
In particular, windows/deps/mkoctfile64 is modified to print its version number
on stdout instead of stderr. This is what the standard mkoctfile does, and it
allows easier capture of the output from Meson.
Sébastien Villemot