Index: llvm/cmake/modules/AddLLVM.cmake
===================================================================
--- llvm/cmake/modules/AddLLVM.cmake
+++ llvm/cmake/modules/AddLLVM.cmake
@@ -2156,6 +2156,7 @@
           RESULT_VARIABLE touch_head_result
           ERROR_QUIET)
         if (NOT touch_head_result EQUAL 0)
+          set(${out_var} "" PARENT_SCOPE)
           return()
         endif()
       endif()
Index: llvm/include/llvm/Support/CMakeLists.txt
===================================================================
--- llvm/include/llvm/Support/CMakeLists.txt
+++ llvm/include/llvm/Support/CMakeLists.txt
@@ -11,7 +11,7 @@
   # A fake version file and is not expected to exist. It is being used to
   # force regeneration of VCSRevision.h for source directory with no write
   # permission available.
-  if (NOT llvm_vc)
+  if (llvm_vc STREQUAL "")
     set(fake_version_inc "${CMAKE_CURRENT_BINARY_DIR}/__FakeVCSRevision.h")
   endif()
 endif()