On GNU/Linux, pick x13 binary from the path if it is there

As a consequence, no need to download the binary on the CI runners, since it is
already there.

.gitlab-ci.yml

@@ -5,9 +5,6 @@ variables:
 before_script:
   - git clone https://git.dynare.org/Dynare/dates
   - git clone https://git.dynare.org/Dynare/m-unit-tests
-  - wget http://www.dynare.org/x13/x13.zip
-  - unzip x13.zip
-  - cp -r binaries/linux externals/x13
 
 testsuite_matlab:
   stage: test

src/initialize_dseries_toolbox.m

@@ -28,7 +28,7 @@ dseries_src_root = strrep(which('initialize_dseries_toolbox'),'initialize_dserie
 % Check that the x13 binary is available
 nox13 = false;
 if isunix()
-    if ~(exist([dseries_src_root '../externals/x13/linux/64/x13as'], 'file') && exist([dseries_src_root '../externals/x13/linux/32/x13as'], 'file'))
+    if system('which x13as') && ~(exist([dseries_src_root '../externals/x13/linux/64/x13as'], 'file') && exist([dseries_src_root '../externals/x13/linux/32/x13as'], 'file'))
         nox13 = true;
     end
 elseif ismac()
@@ -43,7 +43,7 @@ else
     error('Unsupported platform.')
 end
 if nox13
-    warning('X13 binary is not available. You can install it manually by running\nthe function installx13() available in:\n\n\t%s\n', [dseries_src_root(1:end-4), 'externals' filesep() 'x13']);
+    warning('X13 binary is not available.\nIf you are under Debian or Ubuntu, you can install it through your package manager, with ''apt install x13as''.\nOtherwise, you can install it manually by running\nthe function installx13() available in:\n\n\t%s\n', [dseries_src_root(1:end-4), 'externals' filesep() 'x13']);
 end
 
 % Set the subfolders to be added in the path.

src/utilities/x13/select_x13_binary.m

@@ -26,11 +26,16 @@ if ismac()
         x13_binary = sprintf('%s%s%s%s', x13_binary, '32', filesep(), 'x13as');
     end
 elseif isunix()
-    x13_binary = sprintf('%s%s%s', dseries_x13_root, 'linux', filesep());
-    if is64bit()
-        x13_binary = sprintf('%s%s%s%s', x13_binary, '64', filesep(), 'x13as');
+    [status, x13_binary] = system('which x13as');
+    if ~status
+        x13_binary = deblank(x13_binary);
     else
-        x13_binary = sprintf('%s%s%s%s', x13_binary, '32', filesep(), 'x13as');
+        x13_binary = sprintf('%s%s%s', dseries_x13_root, 'linux', filesep());
+        if is64bit()
+            x13_binary = sprintf('%s%s%s%s', x13_binary, '64', filesep(), 'x13as');
+        else
+            x13_binary = sprintf('%s%s%s%s', x13_binary, '32', filesep(), 'x13as');
+        end
     end
 elseif ispc()
     x13_binary = sprintf('%s%s%s', dseries_x13_root, 'windows', filesep());