Enable _FORTIFY_SOURCE=3 by default
    
    Make the _FORTIFY_SOURCE flags configurable so that the command line is
    not cluttered with _FORTIFY_SOURCE definitions and undefines.  Introduce
    a %_fortify_level variable that a package may override by either
    undefining or defining to a specific value.
    
    Also bump the default value to 3, to implement the systemwide proposal
    for Fedora 38:
    
    https://fedoraproject.org/wiki/Changes/Add_FORTIFY_SOURCE%3D3_to_distribution_build_flags