Add ability to use self-compiled x13as on Mac

When I compile Dynare from source on either Linux or Mac, I also compile x13as from source and put it somewhere in my path, e.g. ~/.local/bin/x13as. For Linux we have the ability to get the path to x13as by running which; I don't see a reason why this should not be also useful on a Mac? So this MR does this.

Should also go into 4.7.

src/utilities/x13/select_x13_binary.m

@@ -33,6 +33,12 @@ if ismac()
     else
         x13_binary = sprintf('%s%s%s%s', x13_binary, '32', filesep(), 'x13as');
     end
+    if ~exist(x13_binary, 'file')
+        [status, x13_binary] = system('which x13as');
+        if ~status
+            x13_binary = deblank(x13_binary);
+        end
+    end
 elseif isunix()
     x13_binary = sprintf('%s%s%s', dseries_x13_root, 'linux', filesep());
     if is64bit()