[libcxx][CMake] Move the warning to HandleOutOfTreeLLVM

This currently gives a warning when building libcxx under runtimes.

Differential Revision: https://reviews.llvm.org/D27643

llvm-svn: 289418
diff --git a/libcxx/cmake/Modules/HandleOutOfTreeLLVM.cmake b/libcxx/cmake/Modules/HandleOutOfTreeLLVM.cmake
index 599d8a1..c772143 100644
--- a/libcxx/cmake/Modules/HandleOutOfTreeLLVM.cmake
+++ b/libcxx/cmake/Modules/HandleOutOfTreeLLVM.cmake
@@ -41,6 +41,10 @@
     set(LLVM_CMAKE_PATH "${LLVM_BINARY_DIR}/lib${LLVM_LIBDIR_SUFFIX}/cmake/llvm")
   else()
     set(LLVM_FOUND OFF)
+    message(WARNING "UNSUPPORTED LIBCXX CONFIGURATION DETECTED: "
+                    "llvm-config not found and LLVM_PATH not defined.\n"
+                    "Reconfigure with -DLLVM_CONFIG_PATH=path/to/llvm-config "
+                    "or -DLLVM_PATH=path/to/llvm-source-root.")
     return()
   endif()