[CMake] [OCaml] Add -DLLVM_ENABLE_OCAMLDOC switch

Patch by Michael Gorny.

llvm-svn: 279544
diff --git a/llvm/docs/CMakeLists.txt b/llvm/docs/CMakeLists.txt
index eaa1750..55d867f 100644
--- a/llvm/docs/CMakeLists.txt
+++ b/llvm/docs/CMakeLists.txt
@@ -115,7 +115,7 @@
 endif()
 
 list(FIND LLVM_BINDINGS_LIST ocaml uses_ocaml)
-if( NOT uses_ocaml LESS 0 )
+if( NOT uses_ocaml LESS 0 AND LLVM_ENABLE_OCAMLDOC )
   set(doc_targets
         ocaml_llvm
         ocaml_llvm_all_backends