Merge pull request #10473 from alalek:cmake_version_string_timestamp

cmake/OpenCVUtils.cmake

@@ -519,16 +519,29 @@ endmacro()
 
 if(NOT DEFINED CMAKE_ARGC) # Guard CMake standalone invocations
 
-set(OPENCV_BUILD_INFO_FILE "${CMAKE_BINARY_DIR}/version_string.tmp")
+set(OPENCV_BUILD_INFO_STR "" CACHE INTERNAL "")
-file(REMOVE "${OPENCV_BUILD_INFO_FILE}")
 function(ocv_output_status msg)
   message(STATUS "${msg}")
   string(REPLACE "\\" "\\\\" msg "${msg}")
   string(REPLACE "\"" "\\\"" msg "${msg}")
-  file(APPEND "${OPENCV_BUILD_INFO_FILE}" "\"${msg}\\n\"\n")
+  set(OPENCV_BUILD_INFO_STR "${OPENCV_BUILD_INFO_STR}\"${msg}\\n\"\n" CACHE INTERNAL "")
 endfunction()
 
 macro(ocv_finalize_status)
+  set(OPENCV_BUILD_INFO_FILE "${CMAKE_BINARY_DIR}/version_string.tmp")
+  if(EXISTS "${OPENCV_BUILD_INFO_FILE}")
+    file(READ "${OPENCV_BUILD_INFO_FILE}" __content)
+  else()
+    set(__content "")
+  endif()
+  if("${__content}" STREQUAL "${OPENCV_BUILD_INFO_STR}")
+    #message(STATUS "${OPENCV_BUILD_INFO_FILE} contains the same content")
+  else()
+    file(WRITE "${OPENCV_BUILD_INFO_FILE}" "${OPENCV_BUILD_INFO_STR}")
+  endif()
+  unset(__content)
+  unset(OPENCV_BUILD_INFO_STR CACHE)
+
   if(NOT OPENCV_SKIP_STATUS_FINALIZATION)
     if(DEFINED OPENCV_MODULE_opencv_core_BINARY_DIR)
       execute_process(COMMAND ${CMAKE_COMMAND} -E copy_if_different "${OPENCV_BUILD_INFO_FILE}" "${OPENCV_MODULE_opencv_core_BINARY_DIR}/version_string.inc" OUTPUT_QUIET)