[Docs] Fix Sphinx documentation in CMake check.

Summary:
- `include(AddSphinxTarget)` needs to occur before checking `SPHINX_FOUND`.
- `docs-polly-html` and `docs-polly-man` are now usable again.
- Perhaps we should build docs in the CI as well?

Differential Revision: https://reviews.llvm.org/D33386

llvm-svn: 303549

polly/docs/CMakeLists.txt

@@ -90,8 +90,8 @@ endif()
 endif()
 
 if (LLVM_ENABLE_SPHINX)
+  include(AddSphinxTarget)
   if (SPHINX_FOUND)
-    include(AddSphinxTarget)
     if (${SPHINX_OUTPUT_HTML})
       add_sphinx_target(html polly)
     endif()