Enabling FOLLY_HAVE_LIBUNWIND if libunwind is available (#1501)
Enabling FOLLY_HAVE_LIBUNWIND if libunwind is available (#1501)

Summary:
Previously theFOLLY_HAVE_LIBUNWIND macro would evaluate to false even if LibUnwind was available.

Pull Request resolved: https://github.com/facebook/folly/pull/1501

Reviewed By: yfeldblum

Differential Revision: D25583562

Pulled By: Orvid

fbshipit-source-id: 7dac5afa1f0f24747f4accf2e08b804ab12e416f

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