diff --git a/docs/conf.py b/docs/conf.py
index 356be99..ebc0476 100644
--- a/docs/conf.py
+++ b/docs/conf.py
@@ -20,7 +20,13 @@
 # Add any Sphinx extension module names here, as strings. They can be
 # extensions coming with Sphinx (named 'sphinx.ext.*') or your custom
 # ones.
-extensions = ['sphinx.ext.autosectionlabel', 'sphinxcontrib.plantuml']
+extensions = ['sphinx.ext.autosectionlabel', 'sphinx.ext.intersphinx',
+              'sphinxcontrib.moderncmakedomain', 'sphinxcontrib.plantuml' ]
+
+# Configure intersphinx so that we can refer to external documentation.
+intersphinx_mapping = {
+  'cmake.org': ('https://cmake.org/cmake/help/latest', None)
+}
 
 # Add any paths that contain templates here, relative to this directory.
 templates_path = ['_templates']