[Docs] Use ReadTheDocs theme if available.

Use ReadTheDocs theme for Sphinx if available since it is well
maintained and used by readthedocs.org.

Differential Revision: https://reviews.llvm.org/D33387

llvm-svn: 303550

polly/docs/conf.py

@@ -92,7 +92,13 @@ pygments_style = 'friendly'
 
 # The theme to use for HTML and HTML Help pages.  See the documentation for
 # a list of builtin themes.
-html_theme = 'haiku'
+try:
+    import sphinx_rtd_theme
+    html_theme = "sphinx_rtd_theme"
+    html_theme_path = [sphinx_rtd_theme.get_html_theme_path()]
+except ModuleNotFoundError:
+    html_theme = 'haiku'
+
 
 # Theme options are theme-specific and customize the look and feel of a theme
 # further.  For a list of options available for each theme, see the