cmake: enable threading in samples

- moved code into separate file due issue with handling of CMake policies

samples/CMakeLists.txt

-# Utility function: adds sample executable target with name "example_<group>_<file_name>"
-# Usage:
-#   ocv_define_sample(<output target> <relative filename> <group>)
-function(ocv_define_sample out_target source sub)
-  get_filename_component(name "${source}" NAME_WE)
-  set(the_target "example_${sub}_${name}")
-  add_executable(${the_target} "${source}")
-  set_target_properties(${the_target} PROPERTIES PROJECT_LABEL "(sample) ${name}")
-  if(ENABLE_SOLUTION_FOLDERS)
-    set_target_properties(${the_target} PROPERTIES FOLDER "samples/${sub}")
-  endif()
-  if(WIN32 AND MSVC AND NOT BUILD_SHARED_LIBS)
-    set_target_properties(${the_target} PROPERTIES LINK_FLAGS "/NODEFAULTLIB:atlthunk.lib /NODEFAULTLIB:atlsd.lib /DEBUG")
-  endif()
-  if(WIN32)
-    install(TARGETS ${the_target} RUNTIME DESTINATION "samples/${sub}" COMPONENT samples)
-  endif()
-  # Add single target to build all samples in the group: 'make opencv_samples_cpp'
-  set(parent_target opencv_samples_${sub})
-  if(NOT TARGET ${parent_target})
-    add_custom_target(${parent_target})
-    if(TARGET opencv_samples)
-      add_dependencies(opencv_samples ${parent_target})
-    endif()
-  endif()
-  add_dependencies(${parent_target} ${the_target})
-  set(${out_target} ${the_target} PARENT_SCOPE)
-endfunction()
-
 if(NOT CMAKE_SOURCE_DIR STREQUAL CMAKE_CURRENT_LIST_DIR)
 #===================================================================================================
 #
@@ -34,6 +5,8 @@ if(NOT CMAKE_SOURCE_DIR STREQUAL CMAKE_CURRENT_LIST_DIR)
 #
 #===================================================================================================
 
+include("${CMAKE_CURRENT_LIST_DIR}/samples_utils.cmake")
+
 function(ocv_install_example_src relpath)
   if(INSTALL_C_EXAMPLES)
     file(GLOB files ${ARGN})
@@ -43,6 +16,10 @@ function(ocv_install_example_src relpath)
   endif()
 endfunction()
 
+if((TARGET Threads::Threads OR HAVE_PTHREAD OR MSVC OR APPLE) AND NOT OPENCV_EXAMPLES_DISABLE_THREADS)
+  add_definitions(-DHAVE_THREADS=1)
+endif()
+
 add_subdirectory(cpp)
 add_subdirectory(java/tutorial_code)
 add_subdirectory(dnn)
@@ -94,6 +71,8 @@ option(BUILD_EXAMPLES "Build samples" ON)
 #     │   ├── cpp/
 find_package(OpenCV REQUIRED PATHS "..")
 
+include("${CMAKE_CURRENT_LIST_DIR}/samples_utils.cmake")
+
 function(ocv_install_example_src)
   # not used in this branch
 endfunction()
@@ -125,6 +104,17 @@ endif()
 
 add_definitions(-DDISABLE_OPENCV_24_COMPATIBILITY=1)  # Avoid C-like legacy API
 
+if(OPENCV_EXAMPLES_DISABLE_THREADS)
+  # nothing
+elseif(MSVC OR APPLE)
+  set(HAVE_THREADS 1)
+else()
+  find_package(Threads)
+endif()
+if((TARGET Threads::Threads OR HAVE_THREADS) AND NOT OPENCV_EXAMPLES_DISABLE_THREADS)
+  add_definitions(-DHAVE_THREADS=1)
+endif()
+
 add_subdirectory(cpp)
 if(WIN32)
   add_subdirectory(directx)

samples/samples_utils.cmake

+# Utility function: adds sample executable target with name "example_<group>_<file_name>"
+# Usage:
+#   ocv_define_sample(<output target> <relative filename> <group>)
+function(ocv_define_sample out_target source sub)
+  get_filename_component(name "${source}" NAME_WE)
+  set(the_target "example_${sub}_${name}")
+  add_executable(${the_target} "${source}")
+  if(TARGET Threads::Threads AND NOT OPENCV_EXAMPLES_DISABLE_THREADS)
+    target_link_libraries(${the_target} LINK_PRIVATE Threads::Threads)
+  endif()
+  set_target_properties(${the_target} PROPERTIES PROJECT_LABEL "(sample) ${name}")
+  if(ENABLE_SOLUTION_FOLDERS)
+    set_target_properties(${the_target} PROPERTIES FOLDER "samples/${sub}")
+  endif()
+  if(WIN32 AND MSVC AND NOT BUILD_SHARED_LIBS)
+    set_target_properties(${the_target} PROPERTIES LINK_FLAGS "/NODEFAULTLIB:atlthunk.lib /NODEFAULTLIB:atlsd.lib /DEBUG")
+  endif()
+  if(WIN32)
+    install(TARGETS ${the_target} RUNTIME DESTINATION "samples/${sub}" COMPONENT samples)
+  endif()
+  # Add single target to build all samples in the group: 'make opencv_samples_cpp'
+  set(parent_target opencv_samples_${sub})
+  if(NOT TARGET ${parent_target})
+    add_custom_target(${parent_target})
+    if(TARGET opencv_samples)
+      add_dependencies(opencv_samples ${parent_target})
+    endif()
+  endif()
+  add_dependencies(${parent_target} ${the_target})
+  set(${out_target} ${the_target} PARENT_SCOPE)
+endfunction()