[Polly] [Docs] Fix Sphinx documentation CMake check.

[bollu]

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?

[bollu]

@grosser, @Meinersbur: would it be reasonable to enable building docs in the CI as well? Or is this too much overhead?

I'm no CMake expert, so I put this up for review instead of committing directly.

[bollu]

@philip.pfaffe as well

[grosser]

LGTM.

polly/trunk/docs/CMakeLists.txt

if (LLVM_ENABLE_DOXYGEN)
   if (NOT LLVM_INSTALL_TOOLCHAIN_ONLY)
     install(DIRECTORY ${CMAKE_CURRENT_BINARY_DIR}/doxygen/html
       DESTINATION docs/html)
   endif()
 endif()
 endif()
 
 if (LLVM_ENABLE_SPHINX)
-  if (SPHINX_FOUND)
-    include(AddSphinxTarget)
+  if (SPHINX_FOUND)
     if (${SPHINX_OUTPUT_HTML})
       add_sphinx_target(html polly)
     endif()
     if (${SPHINX_OUTPUT_MAN})
       add_sphinx_target(man polly)
     endif()
   endif()
 endif()