Find the backtrace function using find_package instead of check_function_exists

Summary: When building folly with clang, backtrace was marked as not found. This diff uses `find_package(Backtrace)` instead which allowed us to use the symbolizer libs.

Reviewed By: simpkins

Differential Revision: D19497120

fbshipit-source-id: a8f8b4ffe9a2d53a53b6c7be32ac2a832a840877

CMake/folly-deps.cmake

@@ -136,7 +136,8 @@ if (UNWIND_LIBRARIES)
   list(APPEND FOLLY_LINK_LIBRARIES ${UNWIND_LIBRARIES})
   list(APPEND CMAKE_REQUIRED_LIBRARIES ${UNWIND_LIBRARIES})
 endif()
-check_function_exists(backtrace FOLLY_HAVE_BACKTRACE)
+find_package(Backtrace)
+set(FOLLY_HAVE_BACKTRACE ${Backtrace_FOUND})
 if (FOLLY_HAVE_ELF_H AND FOLLY_HAVE_BACKTRACE AND LIBDWARF_FOUND)
   set(FOLLY_USE_SYMBOLIZER ON)
 endif()