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

build/fbcode_builder/manifests/watchman

@@ -23,3 +23,6 @@ fbcode/eden/fs/service = eden/fs/service
 
 [shipit.strip]
 ^fbcode/eden/fs/service/(?!.*\.thrift|shipit_test_file\.txt)
+
+[cmake.defines]
+ENABLE_EDEN_SUPPORT=ON