cmake: skip missing example targets in libmultiprocess.cmake #35454

bitcoin-core/libmultiprocess#287 is removing the mpcalculator, mpprinter, and mpexample targets, which causes a configuration error when Bitcoin Core is compiled against the updated upstream libmultiprocess:

CMake Error at cmake/libmultiprocess.cmake:37 ... Can not find target to add properties to: mpcalculator

Fix this by iterating over the targets and only calling set_target_properties if the target exists. This is not needed for subtree builds (where upstream changes do not apply directly), but is useful for custom builds with an external libmultiprocess, and is needed to keep Bitcoin Core CI jobs working in the libmultiprocess repository.

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Code Coverage & Benchmarks

For details see: https://corecheck.dev/bitcoin/bitcoin/pulls/35454.

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Conflicts

Reviewers, this pull request conflicts with the following ones:

• #35418 (build: exclude mptest target from compile commands by Sanjana2906)

If you consider this pull request important, please also help to review the conflicting pull requests. Ideally, start with the one that should be merged first.

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