[clangd] add CLANG_ENABLE_CLANGD option to build clangd. Require threads.

CMakeLists.txt

+include(CMakeDependentOption)
+
 add_subdirectory(clang-apply-replacements)
 add_subdirectory(clang-reorder-fields)
 add_subdirectory(modularize)
 add_subdirectory(clang-tidy)
 add_subdirectory(clang-tidy-vs)
 
 add_subdirectory(clang-change-namespace)
 add_subdirectory(clang-doc)
 add_subdirectory(clang-include-fixer)
 add_subdirectory(clang-move)
 add_subdirectory(clang-query)
-add_subdirectory(clangd)
 add_subdirectory(pp-trace)
 add_subdirectory(tool-template)
 
 # Add the common testsuite after all the tools.
 if(CLANG_INCLUDE_TESTS)
 add_subdirectory(test)
 add_subdirectory(unittests)
 endif()
 
 option(CLANG_TOOLS_EXTRA_INCLUDE_DOCS "Generate build targets for the Clang Extra Tools docs."
   ${LLVM_INCLUDE_DOCS})
 if( CLANG_TOOLS_EXTRA_INCLUDE_DOCS )
   add_subdirectory(docs)
 endif()
 
+# clangd has its own CMake tree. It requires threads.
+CMAKE_DEPENDENT_OPTION(CLANG_ENABLE_CLANGD "Build clangd language server" ON
+                       "LLVM_ENABLE_THREADS" OFF)
+if (CLANG_ENABLE_CLANGD)
+  add_subdirectory(clangd)
+endif()