Index: include/llvm/Support/CMakeLists.txt
===================================================================
--- include/llvm/Support/CMakeLists.txt
+++ include/llvm/Support/CMakeLists.txt
@@ -42,6 +42,20 @@
 
 if(DEFINED llvm_vc)
   # Create custom target to generate the VC revision include.
+
+  # If version_inc exists and is empty, remove it so that
+  # get_svn_script will fire.
+  file(WRITE "${version_inc}.txt" "")
+  execute_process(COMMAND ${CMAKE_COMMAND} -E compare_files
+    "${version_inc}.txt" "${version_inc}"
+    RESULT_VARIABLE files_not_equal
+    OUTPUT_QUIET
+    ERROR_QUIET)
+  file(REMOVE "${version_inc}.txt")
+  if(NOT files_not_equal)
+    file(REMOVE "${version_inc}")
+  endif()
+
   add_custom_command(OUTPUT "${version_inc}"
     DEPENDS "${llvm_vc}" "${get_svn_script}"
     COMMAND
@@ -55,7 +69,12 @@
     PROPERTIES GENERATED TRUE
                HEADER_FILE_ONLY TRUE)
 else()
-  file(WRITE "${version_inc}" "")
+  # Ensure ${version_inc} is an empty file.
+  file(WRITE "${version_inc}.txt" "")
+  # Copy the file only if it has changed.
+  execute_process(COMMAND ${CMAKE_COMMAND} -E copy_if_different
+    "${version_inc}.txt" "${version_inc}")
+  file(REMOVE "${version_inc}.txt")
 endif()
 
 add_custom_target(llvm_vcsrevision_h DEPENDS "${version_inc}")