Convert pthreadpool to ExternalProject

[swolchok]

As with the previous PR #12425, I think this is needed to get EXPORT
to work.

[swolchok]

Stack from ghstack (oldest at bottom):

• Use CMake EXPORT #8954
• append to CMAKE_FIND_ROOT_PATH instead of setting executorch_DIR #12618
• Turn BUILD_TESTING off if EXECUTORCH_BUILD_TESTS is off #12566
• Remove bundled_program dep from mpsdelegate #12562
• Build FXDiv as ExternalProject #12561
• format even more CMakeLists.txt files #12560
• Build cpuinfo as ExternalProject #12513
• -> Convert pthreadpool to ExternalProject #12468
• Build XNNPACK as an ExternalProject #12425

[pytorch-bot]

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🧪 See artifacts and rendered test results at hud.pytorch.org/pr/pytorch/executorch/12468

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❌ 5 New Failures, 1 Cancelled Job, 5 Unrelated Failures

As of commit 4fd7372 with merge base b9bb3c1 ():

NEW FAILURES - The following jobs have failed:

• Apple / build-benchmark-app / macos-job (gh)
  RuntimeError: Command bash /Users/runner/work/_temp/exec_script failed with exit code 1
• Apple / build-frameworks-ios / macos-job (gh)
  RuntimeError: Command bash /Users/runner/work/_temp/exec_script failed with exit code 1
• pull / test-eval_llama-mmlu-linux / linux-job (gh)
  RuntimeError: Command docker exec -t 6e09abd82f18508a5b8306b735cea176d1343178f54d29599fdd5fdc584fc51d /exec failed with exit code 1
• pull / test-llava-runner-linux / linux-job (gh)
  RuntimeError: Command docker exec -t 78999d5e9837dd9e040c2cf325d9b937b6eba1f26183927c8f9b1457ef417d65 /exec failed with exit code 2
• pull / unittest-buck / macos / macos-job (gh)
  Can't find 'action.yml', 'action.yaml' or 'Dockerfile' under '/Users/ec2-user/runner/_work/executorch/executorch/test-infra/.github/actions/check-disk-space'. Did you forget to run actions/checkout before running your local action?

CANCELLED JOB - The following job was cancelled. Please retry:

• pull / unittest / macos / macos-job (gh)

BROKEN TRUNK - The following jobs failed but were present on the merge base:

👉 Rebase onto the `viable/strict` branch to avoid these failures

• pull / unittest / linux / linux-job (gh) (trunk failure)
  backends/apple/coreml/test/test_coreml_partitioner.py::TestCoreMLPartitioner::test_ops_to_not_decompose
• pull / unittest-editable / linux / linux-job (gh) (trunk failure)
  backends/apple/coreml/test/test_coreml_partitioner.py::TestCoreMLPartitioner::test_ops_to_not_decompose
• pull / unittest-editable / macos / macos-job (gh) (trunk failure)
  backends/apple/coreml/test/test_coreml_partitioner.py::TestCoreMLPartitioner::test_ops_to_not_decompose
• trunk / unittest-release / linux / linux-job (gh) (trunk failure)
  backends/apple/coreml/test/test_coreml_partitioner.py::TestCoreMLPartitioner::test_ops_to_not_decompose
• trunk / unittest-release / macos / macos-job (gh) (trunk failure)
  backends/apple/coreml/test/test_coreml_partitioner.py::TestCoreMLPartitioner::test_ops_to_not_decompose

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[swolchok]

test-llama-torchao-lowbit is failing with inability to #include <pthreadpool.h>, looks like we need to configure it to install

manywheel build is failing with an XNNPACK issue, unclear why it's not failing on previous rev: gmake[3]: *** No rule to make target 'backends/xnnpack/XNNPACK/install/lib/libXNNPACK.a', needed by '_portable_lib.cpython-310-x86_64-linux-gnu.so'. Stop. https://github.com/pytorch/executorch/actions/runs/16280451716/job/45968809700?pr=12468#step:15:1109

[swolchok]

mac unittests are segfaulting, needs debugging