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Ref. #1713

Ref. #1689

In particular, merge back preprocessor information.

Those had been incorrectly converted to GPL-3+ in commit
1bf81c9f.

[skip ci]

[skip ci]

This means we now require GCC 8.

Boost filesystem library is no longer needed.

Ref preprocessor#16

— Ondra Kamenik has agreed to the relicensing of utils and parser modules to
  GPL
— add DynareTeam’s copyright, since we have made a lot of modifications
— add proper copyright notices in all files

As a consequence, revert commits 2e1f1897 and
5d05adea.

By the way, drop ax_boost_base.m4 which is no longer used.

* Font locking (colorization)
  - all keywords for statements and blocks are now recognized (but not option names)
  - MATLAB-style (single-quoted) strings now recognized
  - improve handling of macroprocessor (but multiline macro commands/exprs not
    yet working)
  - recognize mathematical functions and constants (nan and inf)
  - highlight the pound-sign for model-local variables

* Indentation
  - basic indentation within blocks (offset customizable through option)
  - more elaborate indentation (e.g. multiline equations or statements) not yet
    working

- 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/

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).

Needed because bytecode DLL uses some preprocessor code which is C++14.

They are no longer used in the main codebase.

(cherry picked from commit 1aa042cc)