Refactor test for Octave version

Distinguish between a minimal version required (from a source code perspective,
applicable on all platforms) and an exact version requirement (from a binary
package perspective, in practice used only for our Windows package).

(manually cherry picked from commits c1807778
and 88e89dfb)

matlab/dynare.m

@@ -65,8 +65,20 @@ dynareroot = dynare_config;
 warning_config()
 
 if isoctave
-    if octave_ver_less_than('3.6.0')
-        warning('This version of Dynare has only been tested on Octave 3.6.0 and above. Since your Octave version is older than that, Dynare may fail to run, or give unexpected results. Consider upgrading your Octave installation.');
+    % The supported_octave_version.m file is not in git nor in the source
+    % package, it is manually added in binary packages distributed on dynare.org
+    if exist('supported_octave_version', 'file') && ~strcmp(supported_octave_version, version)
+        skipline()
+        warning(['This version of Octave is not supported. Consider installing ' ...
+                 'version %s of Octave\n' ...
+                 'from www.octave.org, otherwise m files will be used instead ' ...
+                 'of precompiled mex files and some\nfeatures, like solution ' ...
+                 'of models approximated at third order, will not be available.'], supported_octave_version())
+        skipline()
+    elseif octave_ver_less_than('3.6') % Should match the test in mex/build/octave/configure.ac
+        skipline()
+        warning(['This version of Dynare has only been tested on Octave 3.6 and above. Dynare may fail to run or give unexpected result. Consider upgrading your version of Octave.'])
+        skipline()
     end
 else
     if matlab_ver_less_than('7.5')

matlab/dynare_config.m

@@ -87,15 +87,6 @@ if isoctave
     p{end+1} = '/missing/ordeig';
 end
 
-if isoctave && ~isequal(supported_octave_version(), version())
-    skipline()
-    warning(['This version of Octave is not supported. Consider installing ' ...
-             'version %s of Octave,\notherwise m files will be used instead ' ...
-             'of precompiled mex files and some features, like solution\n' ...
-             'of models approximated at third order, will not be available.'], supported_octave_version())
-    skipline()
-end
-
 % ilu is missing in Octave < 4.0
 if isoctave && octave_ver_less_than('4.0')
     p{end+1} = '/missing/ilu';

matlab/supported_octave_version.m

-function v = supported_octave_version()
-
-% Copyright (C) 2017 Dynare Team
-%
-% This file is part of Dynare.
-%
-% Dynare is free software: you can redistribute it and/or modify
-% it under the terms of the GNU General Public License as published by
-% the Free Software Foundation, either version 3 of the License, or
-% (at your option) any later version.
-%
-% Dynare is distributed in the hope that it will be useful,
-% but WITHOUT ANY WARRANTY; without even the implied warranty of
-% MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
-% GNU General Public License for more details.
-%
-% You should have received a copy of the GNU General Public License
-% along with Dynare.  If not, see <http://www.gnu.org/licenses/>.
-
-v = '4.2.1';
\ No newline at end of file