Always use "isoctave" for testing Octave context

matlab/add_path_to_mex_files.m

@@ -21,7 +21,7 @@ if nargin<2
     modifypath = true;
 end
 
-if exist('OCTAVE_VERSION')
+if isoctave
     if ispc() && strcmpi(computer(), 'i686-w64-mingw32')
         mexpath = {[dynareroot '../mex/octave32/']};
     else

matlab/utilities/general/dyn_mex.m

@@ -44,7 +44,7 @@ if ~isempty(Dmex)
     end
 end
 
-if ~exist('OCTAVE_VERSION')
+if ~isoctave
     % Some mex commands are enclosed in an eval(), because otherwise it will make Octave fail
     if ispc
         if strcmp(win_compiler,'msvc')