Do not build protobuf without dnn (#10689)

* Do not build protobuf if dnn is disabled

* Added BUILD_LIST cmake option to the cache

* Moved protobuf to the top level

* Fixed static build

* Fixed world build

* fixup! Fixed world build

cmake/OpenCVFindProtobuf.cmake

@@ -0,0 +1,74 @@
+# If protobuf is found - libprotobuf target is available
+
+ocv_option(WITH_PROTOBUF "Enable libprotobuf" ON)
+ocv_option(BUILD_PROTOBUF "Force to build libprotobuf from sources" ON)
+ocv_option(PROTOBUF_UPDATE_FILES "Force rebuilding .proto files (protoc should be available)" OFF)
+
+set(HAVE_PROTOBUF FALSE)
+
+if(NOT WITH_PROTOBUF)
+  return()
+endif()
+
+function(get_protobuf_version version include)
+  file(STRINGS "${include}/google/protobuf/stubs/common.h" ver REGEX "#define GOOGLE_PROTOBUF_VERSION [0-9]+")
+  string(REGEX MATCHALL "[0-9]+" ver ${ver})
+  math(EXPR major "${ver} / 1000000")
+  math(EXPR minor "${ver} / 1000 % 1000")
+  math(EXPR patch "${ver} % 1000")
+  set(${version} "${major}.${minor}.${patch}" PARENT_SCOPE)
+endfunction()
+
+if(BUILD_PROTOBUF)
+  add_subdirectory("${OpenCV_SOURCE_DIR}/3rdparty/protobuf")
+  set(HAVE_PROTOBUF TRUE)
+else()
+  unset(Protobuf_VERSION CACHE)
+  find_package(Protobuf QUIET)
+
+  # Backwards compatibility
+  # Define camel case versions of input variables
+  foreach(UPPER
+      PROTOBUF_FOUND
+      PROTOBUF_LIBRARY
+      PROTOBUF_INCLUDE_DIR
+      PROTOBUF_VERSION
+      )
+      if (DEFINED ${UPPER})
+          string(REPLACE "PROTOBUF_" "Protobuf_" Camel ${UPPER})
+          if (NOT DEFINED ${Camel})
+              set(${Camel} ${${UPPER}})
+          endif()
+      endif()
+  endforeach()
+  # end of compatibility block
+
+  if(Protobuf_FOUND)
+    if(TARGET protobuf::libprotobuf)
+      add_library(libprotobuf INTERFACE)
+      target_link_libraries(libprotobuf INTERFACE protobuf::libprotobuf)
+    else()
+      add_library(libprotobuf UNKNOWN IMPORTED)
+      set_target_properties(libprotobuf PROPERTIES
+        IMPORTED_LOCATION "${Protobuf_LIBRARY}"
+        INTERFACE_INCLUDE_DIRECTORIES "${Protobuf_INCLUDE_DIR}"
+      )
+      get_protobuf_version(Protobuf_VERSION "${Protobuf_INCLUDE_DIR}")
+    endif()
+    set(HAVE_PROTOBUF TRUE)
+  endif()
+endif()
+
+if(HAVE_PROTOBUF AND PROTOBUF_UPDATE_FILES AND NOT COMMAND PROTOBUF_GENERATE_CPP)
+  find_package(Protobuf QUIET)
+  if(NOT COMMAND PROTOBUF_GENERATE_CPP)
+    message(FATAL_ERROR "PROTOBUF_GENERATE_CPP command is not available")
+  endif()
+endif()
+
+if(HAVE_PROTOBUF)
+  list(APPEND CUSTOM_STATUS protobuf)
+  list(APPEND CUSTOM_STATUS_protobuf "    Protobuf:"
+    BUILD_PROTOBUF THEN "build (${Protobuf_VERSION})"
+    ELSE "${Protobuf_LIBRARY} (${Protobuf_VERSION})")
+endif()