[CMake] Fixing errors in r361513

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.

Reviewers: lebedev.ri, mclow.lists

Subscribers: mgorny, llvm-commits

Tags: #llvm

Differential Revision: https://reviews.llvm.org/D62336

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@361535 91177308-0d34-0410-b5e6-96231b3b80d8

runtimes/CMakeLists.txt

@@ -209,7 +209,7 @@ if(${CMAKE_SOURCE_DIR} STREQUAL ${CMAKE_CURRENT_SOURCE_DIR})
 
 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})