cmake: update timestamp status

- avoid unnecessary rebuilding of OpenCV libraries
- use timestamp of the first launch of CMake
- to return to previous behavior use `-UOPENCV_TIMESTAMP` CMake option

original commit: 3e457952

CMakeLists.txt

@@ -683,6 +683,16 @@ endif()
 # ========================== build platform ==========================
 status("")
 status("  Platform:")
+if(NOT DEFINED OPENCV_TIMESTAMP
+    AND NOT CMAKE_VERSION VERSION_LESS 2.8.11
+    AND NOT BUILD_INFO_SKIP_TIMESTAMP
+)
+  string(TIMESTAMP OPENCV_TIMESTAMP "" UTC)
+  set(OPENCV_TIMESTAMP "${OPENCV_TIMESTAMP}" CACHE STRING "Timestamp of OpenCV build configuration" FORCE)
+endif()
+if(OPENCV_TIMESTAMP)
+  status("    Timestamp:"      ${OPENCV_TIMESTAMP})
+endif()
 status("    Host:"             ${CMAKE_HOST_SYSTEM_NAME} ${CMAKE_HOST_SYSTEM_VERSION} ${CMAKE_HOST_SYSTEM_PROCESSOR})
 if(CMAKE_CROSSCOMPILING)
   status("    Target:"         ${CMAKE_SYSTEM_NAME} ${CMAKE_SYSTEM_VERSION} ${CMAKE_SYSTEM_PROCESSOR})