add option to enable link time optimization

a bit slower to compile for developing so defaults to 'false', but
might be a good idea for the releases.

CMakeLists.txt

@@ -9,6 +9,14 @@ set(CMAKE_CXX_STANDARD 17)
 set(CMAKE_CXX_STANDARD_REQUIRED ON)
 set(CMAKE_CXX_EXTENSIONS OFF)
 
+include(CheckIPOSupported)
+check_ipo_supported(RESULT LinkTimeOptimization OUTPUT IpoError LANGUAGES C CXX)
+if(LinkTimeOptimization)
+  set(CMAKE_INTERPROCEDURAL_OPTIMIZATION TRUE)
+else()
+  message(WARNING "IPO is not supported: ${IpoError}")
+endif()
+
 option(ADDRESS_SANITIZER OFF)
 if(ADDRESS_SANITIZER)
 	message(STATUS "AddressSanitizer ENABLED for non-Release builds")