Use proper output directory when naminging the  libc++ output

git-svn-id: https://llvm.org/svn/llvm-project/libcxx/trunk@251100 91177308-0d34-0410-b5e6-96231b3b80d8
diff --git a/lib/CMakeLists.txt b/lib/CMakeLists.txt
index cdba053..00e875e 100644
--- a/lib/CMakeLists.txt
+++ b/lib/CMakeLists.txt
@@ -164,7 +164,7 @@
   if (LIBCXX_ENABLE_ABI_LINKER_SCRIPT)
     # Replace the libc++ filename with $<TARGET_LINKER_FILE:cxx>
     # after we required CMake 3.0.
-    install(FILES "${CMAKE_CURRENT_BINARY_DIR}/libc++${CMAKE_SHARED_LIBRARY_SUFFIX}"
+    install(FILES "${LIBCXX_LIBRARY_DIR}/libc++${CMAKE_SHARED_LIBRARY_SUFFIX}"
       DESTINATION lib${LIBCXX_LIBDIR_SUFFIX}
       COMPONENT libcxx)
   endif()