Modernize LTO support in CMake

[lemire]

Should be as simple as...

cmake_minimum_required(VERSION 3.9) # CMP0069 NEW
include(CheckIPOSupported)
check_ipo_supported(RESULT ltoresult)
if(ltoresult)
set(CMAKE_INTERPROCEDURAL_OPTIMIZATION TRUE)
endif()

[lemire]

We just need to verify that it works.