Add ChaCha20 compatibility macro (#1146)
Add ChaCha20 compatibility macro (#1146)

Summary:
This can be used to check if ChaCha20 is available for use.

Signed-off-by: Rosen Penev <rosenp@gmail.com>

related: https://github.com/facebookincubator/fizz/pull/27
Pull Request resolved: https://github.com/facebook/folly/pull/1146

Reviewed By: knekritz

Differential Revision: D15583706

Pulled By: yfeldblum

fbshipit-source-id: 1195d623098018992811cd7d6cef72e2366ace19

facebook/folly,facebook/folly,facebook/folly,facebook/folly,facebook/folly