Fix CMake configuration in case CMAKE_CXX_FLAGS is empty.
Fix CMake configuration in case CMAKE_CXX_FLAGS is empty.

Thanks @LB-- for pointing that out!

MiUishadow/magnum,MiUishadow/magnum,MiUishadow/magnum,MiUishadow/magnum,MiUishadow/magnum,MiUishadow/magnum