is64bit.m: test the operating system, not MATLAB’s index size

Closes: #41

src/utilities/is/is64bit.m

 function b = is64bit()
 
-% Returns true iff Matlab 64bit version is used.
+% Returns true iff operating system is 64-bit
+%
+% The test logic is the same as in dynare.m when selecting the preprocessor
+% flavour.
 
-% Copyright (C) 2017 Dynare Team
+% Copyright (C) 2017-2019 Dynare Team
 %
 % This code is free software: you can redistribute it and/or modify
 % it under the terms of the GNU General Public License as published by
@@ -17,5 +20,10 @@ function b = is64bit()
 % You should have received a copy of the GNU General Public License
 % along with Dynare.  If not, see <http://www.gnu.org/licenses/>.
 
-[~, maxsize] = computer();
-b = (maxsize > 2^31);
\ No newline at end of file
+if ispc
+    arch = getenv('PROCESSOR_ARCHITECTURE');
+else
+    [~, arch] = system('uname -m');
+end
+
+b = ~isempty(strfind(arch, '64'));