fix CMake build to link to libunwind correctly again

Summary:
D7642870 accidentally broke linking of folly when libunwind is available.
It change the code to use the contents of `${FOLLY_LINK_LIBRARIES}` in
folly-deps.cmake, but libunwind was added to `${FOLLY_LINK_LIBRARIES}` after
folly-deps.cmake was included, in FollyConfigChecks.cmake.

This moves the check for libunwind to folly-deps.cmake to fix this issue.  All
of the other checks for external libraries are already in folly-deps.cmake.

Reviewed By: yfeldblum

Differential Revision: D7764762

fbshipit-source-id: 922459a7eefcbd92d3d77c02e9c215bb1a5467cc

CMake/FollyConfigChecks.cmake

@@ -223,16 +223,3 @@ if (FOLLY_HAVE_LIBGFLAGS)
     set(FOLLY_GFLAGS_NAMESPACE google)
   endif()
 endif()
-
-set(FOLLY_USE_SYMBOLIZER OFF)
-CHECK_INCLUDE_FILE_CXX(elf.h FOLLY_HAVE_ELF_H)
-find_library(UNWIND_LIBRARIES NAMES unwind)
-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)
-if (FOLLY_HAVE_ELF_H AND FOLLY_HAVE_BACKTRACE AND LIBDWARF_FOUND)
-  set(FOLLY_USE_SYMBOLIZER ON)
-endif()
-message(STATUS "Setting FOLLY_USE_SYMBOLIZER: ${FOLLY_USE_SYMBOLIZER}")

CMake/folly-deps.cmake

@@ -145,6 +145,19 @@ list(APPEND FOLLY_INCLUDE_DIRECTORIES ${LIBURCU_INCLUDE_DIRS})
 list(APPEND FOLLY_LINK_LIBRARIES ${CMAKE_DL_LIBS})
 list(APPEND CMAKE_REQUIRED_LIBRARIES ${CMAKE_DL_LIBS})
 
+set(FOLLY_USE_SYMBOLIZER OFF)
+CHECK_INCLUDE_FILE_CXX(elf.h FOLLY_HAVE_ELF_H)
+find_library(UNWIND_LIBRARIES NAMES unwind)
+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)
+if (FOLLY_HAVE_ELF_H AND FOLLY_HAVE_BACKTRACE AND LIBDWARF_FOUND)
+  set(FOLLY_USE_SYMBOLIZER ON)
+endif()
+message(STATUS "Setting FOLLY_USE_SYMBOLIZER: ${FOLLY_USE_SYMBOLIZER}")
+
 add_library(folly_deps INTERFACE)
 target_include_directories(folly_deps INTERFACE ${FOLLY_INCLUDE_DIRECTORIES})
 target_link_libraries(folly_deps INTERFACE