mcrouter

Part of the difficulty in building mcrouter as a third party stems from the fact that it uses an autotools-based build system. While other Meta OSS projects have adopted to make it easier to build a project with all required dependencies, this tool was primarily written with CMake in mind and unfortunately wouldn't work with mcrouter's build system without making mcrouter-specific accommodations (e.g. explicitly passing in the Thrift compiler path and so on). So, introduce a CMake-based build system for mcrouter and wire it up with . This also allows for the tests (which had been nonfunctional in the autotools build for years) to be run in OSS CI via CMake/CTest, after some fixes for the GH Actions environment and disabling Meta-specific functionality in an OSS context. There are no changes to non-test/build files apart from the elimination of the generated . Use globbing extensively for generating source file lists in CMake files. While this may be a controversial approach, I think in this specific case the benefits outweigh the drawbacks, since this project doesn't use CMake internally so globbing can reduce the amount of times the OSS build needs to be fixed after the fact because a new source file wasn't added to it. To assist reviewing, I've attempted to break this change into reasonable commits. The very first commit simply adds a vendored copy of from a recent Folly checkout, hence the large diff size. I've not yet removed autotools paraphernalia to reduce the amount of changes in this PR—that can be done as a followup.