GitLab

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Also mention in the README that sphinx-doc must be linked, since it is
keg-only.

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It is thus no longer needed to pass the MATLAB_VERSION option to the configure
script when compiling from source.

(cherry picked from commit a69d598b)

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Removed Debian 9 from tested distros as it requires newer version of several packages such as GCC, Octave, bison
Removed Ubuntu 18.04 as it requires newer version of Octave

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It is thus no longer needed to pass the MATLAB_VERSION option to the configure
script when compiling from source.

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This means we now require GCC 8.

Boost filesystem library is no longer needed.

Ref preprocessor#16

The minimum required version of Bison is now 3.2

As a consequence, remove the --enable-openmp option of the configure script.

Ref #1644

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In particular, update explanations about 32- vs 64-bit to include also Octave.

- 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/

GCC 5 supports C++14, but it is still not enough because it does not implement
N4387 that was retroactively applied to the standard in 2015 (see the "Notes"
and "Defect reports" sections of
https://en.cppreference.com/w/cpp/utility/tuple/tuple).

On Windows, this means that a POSIX threads implementation is no longer needed,
since C++11 threads are implemented using native Windows threads.

On GNU/Linux and macOS, POSIX threads are still used under the hood.

A new m4 macro (AX_CXX11_THREAD) is used to add the proper compilation
flags (instead of AX_PTHREAD).

See the discussion in http://eris.dynare.org/pipermail/dev/2018-November/005062.html