sentencepiece: split into multiple outputs, optional gperftools

I am using sentencepiece in a downstream application where I want to
minimize the resulting closures. This commit makes changes to make
sentencepiece a leaner dependency:

- Split the outputs, so that the binaries/headers do not end up in the
  transitive closure in a library dependency.

- Add the `withGPerfTools` option, which is enabled by default, to
  make it possible to disable the gperftools dependency. According to
  the sentencepiece README, this dependency gives a 10-40% performance
  improvement. But in many cases this is overshadowed by the neural
  networks that use piece identifiers as input anyway.

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