title: cmake issue with FetchContent same protobuf dependency between grpc and or-tools
body: <p>im using the FetchContent from cmake to manage my dependencies</p>
  <pre><code>FetchContent_Declare(
          protobuf
          GIT_REPOSITORY https://github.com/protocolbuffers/protobuf.git
          GIT_TAG        v3.13.0
          SOURCE_SUBDIR  cmake
  )
  ## END PROTOBUF ##
  
  ## GRPC ##
  FetchContent_Declare(
          grpc
          GIT_REPOSITORY https://github.com/grpc/grpc.git
          GIT_TAG        v1.33.x
          GIT_PROGRESS   TRUE
  )
  ## END GRPC ##
  
  ## OR-TOOLS ##
  FetchContent_Declare(
          or-tools
          GIT_REPOSITORY https://github.com/google/or-tools.git
          GIT_TAG        master
          GIT_PROGRESS   TRUE
          PATCH_COMMAND git apply &quot;${PROJECT_SOURCE_DIR}/patches/or-tools-protobuf.patch&quot;
  )
  FetchContent_MakeAvailable(protobuf grpc or-tools)
  </code></pre>
  <p>the problem is that both or-tools and grpc use protobuf internally too as FetchContent_Declare so the OR-TOOLS complains about that protobuf already exists</p>
  <pre><code>CMake Error at build/_deps/protobuf-src/cmake/libprotobuf-lite.cmake:64 (add_library):
    add_library cannot create target &quot;libprotobuf-lite&quot; because another target
    with the same name already exists.  The existing target is a static library
    created in source directory
    &quot;/Users/samuaz/Projects/itbit/securities-settlement/cpp/third_party/grpc-src/third_party/protobuf/cmake&quot;.
    See documentation for policy CMP0002 for more details.
  Call Stack (most recent call first):
    build/_deps/protobuf-src/cmake/CMakeLists.txt:250 (include) 
  </code></pre>
  <p>if I try to disable the protobuf for or-tools set(BUILD_Protobuf OFF), the or-tools complan aboud package protobuf is not found</p>
  <pre><code>--   No package 'protobuf' found
  CMake Error at /Users/samuaz/Library/Application Support/JetBrains/Toolbox/apps/CLion/ch-0/202.6948.80/CLion.app/Contents/bin/cmake/mac/share/cmake-3.17/Modules/FindPkgConfig.cmake:497 (message):
    A required package was not found
  </code></pre>
  <p>how can avoid this issue or make the package protobuf available from the previous protobuf FetchContent_Declare?</p>
  <p>thanks!</p>