```cmake
# Copyright 2020 Google LLC
#
# 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)

if(TARGET firestore)
  return()
endif()

set(version Firestore-1.15.0)

ExternalProject_Add(
  firestore

  DOWNLOAD_DIR ${FIREBASE_DOWNLOAD_DIR}
  URL https://github.com/firebase/firebase-ios-sdk/archive/${version}.tar.gz

  PREFIX ${PROJECT_BINARY_DIR}

  CONFIGURE_COMMAND ""
  BUILD_COMMAND ""
  INSTALL_COMMAND ""
  TEST_COMMAND ""
)
```

Fix open source C++ Firestore building with -DFIREBASE_IOS_BUILD_TESTS=OFF.
user
```cmake
# Copyright 2020 Google LLC
#
# 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)

if(TARGET firestore)
  return()
endif()

# Pin to the first revision that includes these changes:
# https://github.com/firebase/firebase-ios-sdk/pull/5807.
#
# This should be released with the next version after Firestore-1.15.0.
set(version 05026f3df92cdee7b91fccc72a7195be2618acdf)

ExternalProject_Add(
  firestore

  DOWNLOAD_DIR ${FIREBASE_DOWNLOAD_DIR}
  URL https://github.com/firebase/firebase-ios-sdk/archive/${version}.tar.gz

  PREFIX ${PROJECT_BINARY_DIR}

  CONFIGURE_COMMAND ""
  BUILD_COMMAND ""
  INSTALL_COMMAND ""
  TEST_COMMAND ""
)
```
assistant