cmake: use CMAKE_INTERPROCEDURAL_OPTIMIZATION for LTO

CMakeLists.txt

@@ -1,5 +1,6 @@
 cmake_minimum_required (VERSION 3.1)
 project (opendht)
+cmake_policy (SET CMP0069 NEW)
 set (opendht_VERSION_MAJOR 1)
 set (opendht_VERSION_MINOR 7.4)
 set (opendht_VERSION ${opendht_VERSION_MAJOR}.${opendht_VERSION_MINOR})
@@ -81,13 +82,7 @@ else ()
     add_definitions(-DOPENDHT_LOG=false)
 endif()
 if (OPENDHT_LTO)
-    set (CMAKE_CXX_FLAGS "${CMAKE_CXX_FLAGS} -flto")
-    if (CMAKE_COMPILER_IS_GNUCC)
-        set (CMAKE_CXX_FLAGS "${CMAKE_CXX_FLAGS} -fuse-linker-plugin")
-        set (CMAKE_AR        "gcc-ar")
-        set (CMAKE_NM        "gcc-nm")
-        set (CMAKE_RANLIB    "gcc-ranlib")
-    endif ()
+    set(CMAKE_INTERPROCEDURAL_OPTIMIZATION TRUE)
 endif ()
 
 if (MSGPACK_INCLUDE_DIRS)
@@ -246,9 +241,6 @@ if (OPENDHT_STATIC)
         ${opendht_HEADERS}
     )
     set_target_properties (opendht-static PROPERTIES OUTPUT_NAME "opendht")
-    if (OPENDHT_LTO)
-        set_property(TARGET opendht-static APPEND_STRING PROPERTY LINK_FLAGS "-flto -fuse-linker-plugin")
-    endif ()
     if (OPENDHT_ARGON2)
         target_include_directories(opendht-static SYSTEM PRIVATE argon2)
     else ()
@@ -268,9 +260,6 @@ if (OPENDHT_SHARED)
     set_target_properties (opendht PROPERTIES IMPORT_SUFFIX "_import.lib")
     set_target_properties (opendht PROPERTIES SOVERSION ${opendht_VERSION_MAJOR} VERSION ${opendht_VERSION})
     target_compile_definitions(opendht PRIVATE OPENDHT_BUILD)
-    if (OPENDHT_LTO)
-        set_property(TARGET opendht APPEND_STRING PROPERTY LINK_FLAGS -flto -fuse-linker-plugin)
-    endif ()
     if (OPENDHT_ARGON2)
         target_include_directories(opendht SYSTEM PRIVATE argon2)
     else ()