Documentation: Add linkcheck target

Sphinx provides a run-target to verify external links specified in the
documentation. This requires an active connection to be able to validate
the links.

Add a meson target to integrate the linkcheck facility into our build
and test system.

Signed-off-by: Kieran Bingham <kieran.bingham@ideasonboard.com>
Reviewed-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>

Documentation/meson.build

@@ -62,4 +62,10 @@ if sphinx.found()
                   build_by_default : true,
                   install : true,
                   install_dir : doc_install_dir)
+
+    custom_target('documentation-linkcheck',
+                  command: [sphinx, '-W', '-b', 'linkcheck', meson.current_source_dir(), '@OUTPUT@'],
+                  build_always_stale: true,
+                  input: docs_sources,
+                  output: 'linkcheck')
 endif