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

CMake/folly-deps.cmake

@@ -156,6 +156,9 @@ endif ()
 find_package(LibUnwind)
 list(APPEND FOLLY_LINK_LIBRARIES ${LIBUNWIND_LIBRARIES})
 list(APPEND FOLLY_INCLUDE_DIRECTORIES ${LIBUNWIND_INCLUDE_DIRS})
+if (LIBUNWIND_FOUND)
+  set(FOLLY_HAVE_LIBUNWIND ON)
+endif()
 
 cmake_push_check_state()
 set(CMAKE_REQUIRED_DEFINITIONS -D_XOPEN_SOURCE)