[CMake] Fixing errors in r361513

[beanz]

Summary

I somehow messed this up. libcxx appends the subdirectories itself, so we don't need to add them here.

Also, r361513 broke the "projects" build of libcxx because it always included the extra targets.

[lebedev.ri]

Thanks, works

[mclow.lists]

This change fixed my broken build on Ubuntu.

llvm/runtimes/CMakeLists.txt

else()
       configure_file(
         ${CMAKE_CURRENT_SOURCE_DIR}/Components.cmake.in
         ${LLVM_BINARY_DIR}/runtimes/Components.cmake)
     endif()
   endif()
 
 else() # if this is included from LLVM's CMake
   include(LLVMExternalProjectUtils)
-  if (LLVM_EXTERNAL_LIBCXX_SOURCE_DIR)
+  if (LLVM_EXTERNAL_LIBCXX_SOURCE_DIR AND "libcxx" IN_LIST LLVM_ENABLE_RUNTIMES)
     set(LIBCXX_HEADER_DIR ${LLVM_INCLUDE_DIR}/c++/v1/)
     set(CXX_HEADER_TARGET runtime-libcxx-headers)
     add_subdirectory(${LLVM_EXTERNAL_LIBCXX_SOURCE_DIR}/include ${CXX_HEADER_TARGET})
   endif()
 
   if(NOT LLVM_BUILD_RUNTIMES)
     set(EXTRA_ARGS EXCLUDE_FROM_ALL)
   endif()