Define namespace prefixed alias for cpuinfo in the CMake build

[petrhosek]

This allows projects that depend on cpuinfo to use namespace qualified target name regardless of whether they consume cpuinfo through add_subdirectory or find_package.

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[petrhosek]

Thank you for the review, would it be possible to merge this PR?

[Maratyszcza]

I don't have write access to the repo. @malfet PTAL

[malfet]

@Maratyszcza does the change look good to you? @petrhosek can you please share link to CMake doc explaining what namespace prefixes are and what version of cmake they got introduced

[petrhosek]

It's a common convention in modern CMake to provide aliases for each installed library to ensure that the library can be consumes via target_link(<pkg>::<target_name>) both if the project is used via find_package(<pkg>) or via add_subdirectory.

See for example https://github.com/abseil/abseil-cpp/blob/d2c5297a3c3948de765100cb7e5cccca1210d23c/CMake/AbseilHelpers.cmake#L329. This can avoid similar constructs in downstream projects such as https://github.com/google/ruy/blob/368db7123bf3178c92c8502696c6865fb07bfcda/CMakeLists.txt#L85.

There's an ongoing discussion about how to supports this directly in CMake, see https://gitlab.kitware.com/cmake/cmake/-/issues/22687

[malfet]

Thank you for the explanation, looks good to me

[Maratyszcza]

cc @bjacob