Skip CMake configure when build directory already exists

[kirklandsign]

Summary

Check for CMakeCache.txt before running cmake configure. If the build directory is already configured, go straight to cmake --build, which auto-reconfigures when CMakeLists.txt files change. Saves ~37s per ABI on incremental builds. To force a full reconfigure, delete the cmake-out-android-* directories.

Test plan

CI

[pytorch-bot]

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❌ 4 New Failures, 1 Unrelated Failure

As of commit 0950708 with merge base bf0cd1c ():

NEW FAILURES - The following jobs have failed:

• pull / test-mediatek-models-linux / linux-job (gh)
  RuntimeError: Command docker exec -t 52089e8fb4186a7cf21b3a5af4f848e634084948e829eaadfabef912aae0ee2e /exec failed with exit code 2
• pull / test-multimodal-linux (gemma3-4b) / linux-job (gh)
  RuntimeError: Command docker exec -t 3672e54adaf278d0059b954b61d4fd55288afb0415daff6a6b6da48b182caed1 /exec failed with exit code 139
• pull / test-openvino-linux / linux-job (gh)
  RuntimeError: Command docker exec -t b742119a5c41bedfbb08a1168ec9b2a0c63b14a732281196a96a340dbd200b64 /exec failed with exit code 1
• pull / unittest-buck / linux / linux-job (gh)
  RuntimeError: Command docker exec -t e2d766ccd9a837b06c1583949342481352fdc8f15462fc9a89ab65a7b496eca1 /exec failed with exit code 3

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

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

• pull / unittest-buck / macos / macos-job (gh) (trunk failure)
  RuntimeError: Command bash /Users/ec2-user/runner/_work/_temp/exec_script failed with exit code 3

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[Copilot]

Pull request overview

Updates the Android native build script to avoid re-running CMake configure when an existing build directory is already configured, improving incremental build times for repeated ABI builds.

Changes:

• Add a CMakeCache.txt existence check to skip cmake configure on subsequent runs.
• Proceed directly to cmake --build when the build directory appears configured.

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[Copilot]

Skipping the configure step based solely on CMakeCache.txt means changes to any of the -D options passed here (e.g., EXECUTORCH_BUILD_QNN/NEURON/VULKAN, CMAKE_BUILD_TYPE) will be silently ignored on subsequent runs; cmake --build will not update cached values unless a configure is re-run. Consider re-running configure when key cache entries don’t match the current env (at minimum CMAKE_BUILD_TYPE), or add a supported override (e.g., EXECUTORCH_FORCE_CMAKE_CONFIGURE=1) to force configure without requiring users to delete the build directory.