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.
