Re Dynare/dynare#1490.

Incidentally, fix a memory leak related to DynamicModelAC instantiation, using
a std::unique_ptr (see #1490).

Otherwise the DLL is dynamically linked against libwinpthread. This regression
was introduced by commit 671fe4bc.

- Use the -static flag when linking Dynare++, so that shipping libquadmath and
  libgcc DLL in the installer is no longer needed.

- Use AM_CXXFLAGS and AM_LDFLAGS variables for changing flags, since CXXFLAGS
  and LDFLAGS are user variables. Also, this avoids passing these flags down to
  configure scripts in subdirectories.

- Check for the SZIP library in the configure test for the MatIO, this is
  needed under MSYS2.

- Statically link MatIO and GSL in MEX files for MATLAB, this is needed under
  MSYS2.

Adapt the k-order DLL as a consequence.

The gmres() function has been implemented (actually since Octave 3.4.1!).

Adapt bytecode DLL as a consequence.

The bug would show at order 3 when the last output argument (derivs) is not
requested (in practice every 3rd order solution without pruning). The DLL would
still attempt to write into it, causing an invalid memory access.

- M and MEX files are now under +${MODELNAME}/
- bytecode, C source and JSON now under ${MODELNAME}/model/

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

It's actually not needed. The QZ complex decomposition can be obtained in
Octave on real matrices by casting them to the complex type.

These functions are all included in Octave 4.2.

This breaks the matrix.h of older MATLABs (e.g. R2009a). Also see <uchar.h>.

This API was introduced in MATLAB 9.4 (R2018a), because the internal
representation of complex numbers has changed.

(cherry picked from commit ab7acc49e30fbcb292d63d58281581a308716b4c)

(cherry picked from commit fa5dd681abcb3243a5ee9be1f71fdfc51bced1cd)

- Memory allocation is checked
- The amount of memory allocated for conditional forecast is corrected

... Which are not in standard C/C++ (otherwise compilation of the mex for Octave
with -std=c++11 fails).

mjdgges: fix bug where qz_criterium was not taken into account if a threshold argument is also passed

Fixed by https://sourceware.org/bugzilla/show_bug.cgi?id=19439

closes issue #1250