enable EdenFS support by default in open source build
enable EdenFS support by default in open source build

Summary:
The open source builds have been failing for a while, because
test_capabilities assumes the EdenFS watcher is available.

The ENABLE_EDEN_SUPPORT option was originally added to reduce build
times in the open source build, but we already unconditionally build
fbthrift and fb303. To see if this unblocks the releases, default this
option on. Eventually, we should remove it, and if open source builds
are a problem, we should address that directly, perhaps by adding a
non-Thrift protocol to communicate with EdenFS.

Reviewed By: fanzeyi

Differential Revision: D31635078

fbshipit-source-id: 4369e7a7270a7fdffc8d6df009203d553be6df69

facebook/watchman,facebook/watchman,facebook/watchman,nodakai/watchman,facebook/watchman,nodakai/watchman,nodakai/watchman,nodakai/watchman,nodakai/watchman,facebook/watchman,facebook/watchman,nodakai/watchman,nodakai/watchman,nodakai/watchman,facebook/watchman,facebook/watchman,nodakai/watchman,facebook/watchman