GitLab

- integrate with autoconf/automake, so that "make html pdf" now build the
  manual
- remove the old texinfo manual
- the PDF manual is now called dynare-manual.pdf (Closes #1543)
- add a Gitlab CI job that updates the online unstable manual under
  www.dynare.org/manual-unstable/

- TL_RAISE now unconditionally raises an exception
- rathe use TL_RAISE_IF at some places, to save a test in non-debug mode

Previously it was returning a pointer, which is non-standard for containers,
and was leading to ugly dereferencing everywhere.

Also improve on the comments.

- fix C mode
- add C++ mode
- add Octave mode

As a consequence, the singleton implementation has to be made thread-safe.

Also implement the singleton pattern using a namespace, rather than a static
instance.

There were two implementations of integer exponentiation. Merge them into a new
file under utils/cc/.

By the way, optimize it using exponentiation by squaring.

[skip ci]

... Not a copy of all the files.

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The gamrnd fallback under matlab/missing/stats/ does not work under Octave
because the +gamrnd/ folder is not accessible (it has the same name as the
function, which does not work under Octave).

Instead of fixing this, rather make the statistics toolbox a requirement, since
anyways it is very easy to obtain under Octave.

Accordingly:
- do not try to run the unit tests of matlab/missing/stats/ under Octave
- merge the matlab/missing/stats-matlab/ into matlab/missing/stats/, since this
  directory is now MATLAB-only.

Also:
- move matlab/distributions/+gamrnd/ under matlab/missing/stats/ for
  consistency
- in the manual: remove obsolete link to Octave downloads on the Dynare
  website; update URL of Octave Forge

Closes #1638

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