CMake: Fix docs-llvm-man target when clang+llvm is in the same source tree

[tstellar]

Summary

This was broken by r302499. Configuring with -DLLVM_BUILD_DOCS=ON would
cause the docs-llvm-man target not to be created.

[anemet]

Would be good to include the analysis for the failure in the log/description.

Looks good according to my understanding of the problem.

llvm/trunk/cmake/modules/AddSphinxTarget.cmake

 
 # Create sphinx target
-if (LLVM_ENABLE_SPHINX AND NOT TARGET sphinx)
+if (LLVM_ENABLE_SPHINX)
   message(STATUS "Sphinx enabled.")
   find_package(Sphinx REQUIRED)
-  if (LLVM_BUILD_DOCS)
+  if (LLVM_BUILD_DOCS AND NOT TARGET sphinx)
     add_custom_target(sphinx ALL)
   endif()
 else()
   message(STATUS "Sphinx disabled.")
 endif()
 
 
 # Handy function for creating the different Sphinx targets.