Merge pull request #16619 from alalek:cmake_custom_cxx_standard_master

* cmake: allow customization of CMAKE_CXX_STANDARD value

* cmake: extra skip flag OPENCV_SKIP_CMAKE_CXX_STANDARD

* cmake: dump CMAKE_CXX_STANDARD value

- compiler option is missing in dumped flags

CMakeLists.txt

@@ -1049,6 +1049,9 @@ string(STRIP "${OPENCV_COMPILER_STR}" OPENCV_COMPILER_STR)
 status("")
 status("  C/C++:")
 status("    Built as dynamic libs?:" BUILD_SHARED_LIBS THEN YES ELSE NO)
+if(DEFINED CMAKE_CXX_STANDARD AND CMAKE_CXX_STANDARD)
+  status("    C++ standard:"           "${CMAKE_CXX_STANDARD}")
+endif()
 status("    C++ Compiler:"           ${OPENCV_COMPILER_STR})
 status("    C++ flags (Release):"    ${CMAKE_CXX_FLAGS} ${CMAKE_CXX_FLAGS_RELEASE})
 status("    C++ flags (Debug):"      ${CMAKE_CXX_FLAGS} ${CMAKE_CXX_FLAGS_DEBUG})

cmake/OpenCVDetectCXXCompiler.cmake

@@ -192,11 +192,13 @@ if(CMAKE_VERSION VERSION_LESS "3.1")
   endforeach()
 endif()
 
-set(CMAKE_CXX_STANDARD 11)
-set(CMAKE_CXX_STANDARD_REQUIRED TRUE)
-set(CMAKE_CXX_EXTENSIONS OFF) # use -std=c++11 instead of -std=gnu++11
-if(CMAKE_CXX11_COMPILE_FEATURES)
-  set(HAVE_CXX11 ON)
+if(NOT OPENCV_SKIP_CMAKE_CXX_STANDARD)
+  ocv_update(CMAKE_CXX_STANDARD 11)
+  ocv_update(CMAKE_CXX_STANDARD_REQUIRED TRUE)
+  ocv_update(CMAKE_CXX_EXTENSIONS OFF) # use -std=c++11 instead of -std=gnu++11
+  if(CMAKE_CXX11_COMPILE_FEATURES)
+    set(HAVE_CXX11 ON)
+  endif()
 endif()
 if(NOT HAVE_CXX11)
   ocv_check_compiler_flag(CXX "" HAVE_CXX11 "${OpenCV_SOURCE_DIR}/cmake/checks/cxx11.cpp")