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

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

Reviewers: anemet, beanz

Reviewed By: anemet

Subscribers: llvm-commits, mgorny

Differential Revision: https://reviews.llvm.org/D33146

llvm-svn: 303042
diff --git a/llvm/cmake/modules/AddSphinxTarget.cmake b/llvm/cmake/modules/AddSphinxTarget.cmake
index c3a676d..4540c5c 100644
--- a/llvm/cmake/modules/AddSphinxTarget.cmake
+++ b/llvm/cmake/modules/AddSphinxTarget.cmake
@@ -1,9 +1,9 @@
 
 # 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()