Stop hardcoding LLVM version as 12, instead read it from external/llvm-project. In81e20472a0
the location of the LLVM_VERSION_MAJOR definition was moved. This patch accounts for that by extracting the version from the new location if it exists. If the new file is not present, it falls back to the old location. Backport-to: 24.2 Tested-by: Antonio Ospite <antonio.ospite@collabora.com> Tested-by: Alessandro Astone <alessandro.astone@canonical.com> Reviewed-by: Mauro Rossi <issor.oruam@gmail.com> Signed-off-by: Valentine Burley <valentine.burley@gmail.com> Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/30598> (cherry picked from commit71a45e4e0f
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