chore: import upstream snapshot with attribution

cmake/external/threadpool.cmake

@@ -0,0 +1,43 @@
+# Copyright (c) 2018 PaddlePaddle Authors. All Rights Reserved.
+#
+# Licensed under the Apache License, Version 2.0 (the "License");
+# you may not use this file except in compliance with the License.
+# You may obtain a copy of the License at
+#
+# http://www.apache.org/licenses/LICENSE-2.0
+#
+# Unless required by applicable law or agreed to in writing, software
+# distributed under the License is distributed on an "AS IS" BASIS,
+# WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+# See the License for the specific language governing permissions and
+# limitations under the License.
+
+include(ExternalProject)
+
+set(THREADPOOL_PREFIX_DIR ${THIRD_PARTY_PATH}/threadpool)
+set(THREADPOOL_TAG 9a42ec1329f259a5f4881a291db1dcb8f2ad9040)
+set(SOURCE_DIR ${PADDLE_SOURCE_DIR}/third_party/threadpool)
+set(THREADPOOL_INCLUDE_DIR ${SOURCE_DIR})
+include_directories(${THREADPOOL_INCLUDE_DIR})
+
+file(TO_NATIVE_PATH ${PADDLE_SOURCE_DIR}/patches/threadpool/ThreadPool.h.patch
+     threadpool_cxx20_patch)
+
+ExternalProject_Add(
+  extern_threadpool
+  ${EXTERNAL_PROJECT_LOG_ARGS}
+  SOURCE_DIR ${SOURCE_DIR}
+  PREFIX ${THREADPOOL_PREFIX_DIR}
+  UPDATE_COMMAND ""
+  PATCH_COMMAND git checkout -- .
+  COMMAND git checkout ${THREADPOOL_TAG}
+  COMMAND git apply --check ${threadpool_cxx20_patch}
+  COMMAND git apply ${threadpool_cxx20_patch}
+  CONFIGURE_COMMAND ""
+  BUILD_COMMAND ""
+  INSTALL_COMMAND ""
+  TEST_COMMAND "")
+
+add_library(simple_threadpool INTERFACE)
+
+add_dependencies(simple_threadpool extern_threadpool)