Users should be the ones to choose to add -flto or not and how. The test
may also cause unnecessary noise wrt https://bugs.gentoo.org/830950
--- a/src/CMakeLists.txt
+++ b/src/CMakeLists.txt
@@ -42,13 +42,4 @@
 
 # enable global link time optimization
-cmake_policy(SET CMP0069 NEW)
-set(CMAKE_POLICY_DEFAULT_CMP0069 NEW)
-include(CheckIPOSupported)
-check_ipo_supported(RESULT ipo_supported OUTPUT ipo_supported_output)
-if(ipo_supported)
-    set(CMAKE_INTERPROCEDURAL_OPTIMIZATION TRUE)
-else()
-    message(WARNING "IPO is not supported: ${ipo_supported_output}")
-endif()
 
 if(USE_SYSTEM_NCNN)