Facebook BUCK support

Is there any chance of BUCK files being released for folly?

Some old commits and file messages indicate that BUCK might be being used inside Facebook to build folly. (As another example, see reference to '//folly:folly', '//folly/io/async:async' in the buck docs [here](https://buckbuild.com/rule/cxx_precompiled_header.html).) If there are internal BUCK files, it'd be invaluable to have them in the open source version. That way, those of us using buck on the outside can more easily make use of folly.

If anyone's figured out another way to add folly to a buck project, I'd love to hear how.

Review the issue's references to Facebook's internal BUCK files and the Buck documentation examples for //folly:folly and //folly/io/async:async. Determine the required open-source BUCK coverage and verify that an external Buck project can build folly; done means the relevant BUCK files are available and usable outside Facebook.

Written by the indexing model from the issue text.