When using add_definitions erroneous flags are passed to dll resource compiler under MinGW. This breaks the build for anything linked to dlib, using a dll rc and MinGW.

The proper way is to append to CMAKE_CXX_FLAGS.

For CMake 2.8.12 and higher the proper way is add_compile_options(-Wreturn-type) OR target_compile_options(dlib PRIVATE -Wreturn-type)