Closes: #1750

— FCFLAGS needs to contain the compilation flags used when compiling Octave,
  otherwise it fails at configure stage when looking for gfortran
— Explicitly add -L$OCTLIBDIR, because with Octave 5 it is no longer there, and
  on Fedora the Octave libraries are no in the default linker search path

This is a Fortran 2008 rewrite of disclyap_fast.m.

Closes: #1737

If something was already in LDFLAGS, it was concatenated without a whitespace,
leading to failure.

Ref. #1713

Automake actually provides such rules.

By the same token, remove unused support for Fortran 77.

Sébastien Villemot authored Jun 28, 2019 and Stéphane Adjemian committed Dec 20, 2019

It applies the approximated policy function to a set of particles, using
Dynare++ routines.

There is support for parallelization, using Dynare++ multithreading
model (itself based on C++11 threads; we don’t use OpenMP because it is
incompatible with MKL). For the time being, default to a single thread. This
should be later refined through empirical testing.

macOS packaging: compile against the version of Octave distributed via https://octave-app.org (currently 4.4.1)

The changes in 8065e9d4 were not working as
intended, because AC_CHECK_PROG expect values and not actions. Hence
AC_MSG_ERROR was not properly executed.

By the way, simplify some test conditions using && instead of if/then/fi
blocks.

In particular, if either MATLAB or Octave is missing, one needs to pass either
--disable-matlab or --disable-octave.

Moreover, several new configure flags have been introduced for disabling some
components:
--disable-doc
--disable-dynare++
--disable-mex-dynare++
--disable-mex-ms-sbvar
--disable-mex-kalman-steady-state

This means we now require GCC 8.

Boost filesystem library is no longer needed.

Ref preprocessor#16

The MEX files are built out-of-tree (because we want to do them in parallel).
This would create a potential race condition if several builds want to create
the symlinks under mex/matlab/ or mex/octave/.

The solution is to disable those symlinks for out-of-tree builds.

The scripts are based the former “dynare-build” project. They have been
overhauled and simplified.

Building a Windows package (both installer and zip archive) is as easy as
running “make -C windows” (provided the right Debian packages are installed,
use the “windows/install-packages.sh” script for that purpose).

The layout of MEX files for Octave in the package has been
changed (mex/octave/win32/ and mex/octave/win64/ instead of mex/octave32/ and
mex/octave/), for consistency with MATLAB MEX.

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

It constructs the stacked residuals and jacobian of the perfect foresight
problem.

It is an almost perfect replacement for the perfect_foresight_problem.m
routine, while being much more efficient.

Note however that the DLL never return complex numbers (it instead puts NaNs at
the place where there would have been complex). This may create problems for
some MOD files; the algorithms will need to be adapted to use a more
line-search method.

Various modernizations and simplifications.

Also remove a workaround for a LAPACK bug in DGGES (the VSL argument was
apparently referenced even though JOBVSL="N"). Hopefully the bug has been fixed
everywhere now.

Adapt the code accordingly.

We now use a symbolic link farm to make source file accessible from build
directories.

Closes #466