LLVM_FOUND isn't always set, so just test if llvm_setup_rpath() is
available instead.

git-svn-id: https://llvm.org/svn/llvm-project/libcxx/trunk@323599 91177308-0d34-0410-b5e6-96231b3b80d8
diff --git a/lib/CMakeLists.txt b/lib/CMakeLists.txt
index e7de4e3..5d71111 100644
--- a/lib/CMakeLists.txt
+++ b/lib/CMakeLists.txt
@@ -222,7 +222,7 @@
 # Build the shared library.
 if (LIBCXX_ENABLE_SHARED)
   add_library(cxx_shared SHARED $<TARGET_OBJECTS:cxx_objects>)
-  if(LLVM_FOUND)
+  if(COMMAND llvm_setup_rpath)
     llvm_setup_rpath(cxx_shared)
   endif()
   target_link_libraries(cxx_shared ${LIBCXX_LIBRARIES})