support lto (#6140)

* support lto

* use relative path

* add another variable to control lto

* remove unnecessary code

cmake/ConfigureCompiler.cmake

@@ -340,9 +340,19 @@ else()
     set(DTRACE_PROBES 1)
   endif()
 
-  if(CMAKE_COMPILER_IS_GNUCXX)
-    set(USE_LTO OFF CACHE BOOL "Do link time optimization")
-    if (USE_LTO)
+  set(USE_LTO OFF CACHE BOOL "Do link time optimization")
+  if (USE_LTO)
+    if (CLANG)
+      set(CLANG_LTO_STRATEGY "Thin" CACHE STRING "LLVM LTO strategy (Thin, or Full)")
+      if (CLANG_LTO_STRATEGY STREQUAL "Full")
+        add_compile_options($<$<CONFIG:Release>:-flto=full>)
+      else()
+        add_compile_options($<$<CONFIG:Release>:-flto=thin>)
+      endif()
+      set(CMAKE_RANLIB "llvm-ranlib")
+      set(CMAKE_AR "llvm-ar")
+    endif()
+    if(CMAKE_COMPILER_IS_GNUCXX)
       add_compile_options($<$<CONFIG:Release>:-flto>)
       set(CMAKE_AR  "gcc-ar")
       set(CMAKE_C_ARCHIVE_CREATE "<CMAKE_AR> qcs <TARGET> <LINK_FLAGS> <OBJECTS>")