Index: include/llvm/Support/CMakeLists.txt
===================================================================
--- include/llvm/Support/CMakeLists.txt
+++ include/llvm/Support/CMakeLists.txt
@@ -55,7 +55,9 @@
     PROPERTIES GENERATED TRUE
                HEADER_FILE_ONLY TRUE)
 else()
-  file(WRITE "${version_inc}" "")
+  if(NOT EXISTS "${version_inc}")
+    file(WRITE "${version_inc}" "")
+  endif()
 endif()
 
 add_custom_target(llvm_vcsrevision_h DEPENDS "${version_inc}")