watchman: add cmake option to select whether we build with eden
watchman: add cmake option to select whether we build with eden

Summary:
This makes it possible to disable eden (and thus thrift) support.

I've defaulted this to off so that we don't break the existing
watchman CI.

Reviewed By: simpkins, strager

Differential Revision: D14726767

fbshipit-source-id: 0f4d0597d901a91850f1ba6d71609c059c064c22

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