Undefine _auto_set_build_flags
    
    Fedora 36 and on will automatically set and export build flags during %build,
    %check, and %install phases of RPM builds [1]. This is unnecessary for build2
    since those flags are already set by the config.* options, so undefine
    _auto_set_build_flags to disable this.
    
     * build2.spec: undefine _auto_set_build_flags
    
    [1] https://fedoraproject.org/wiki/Changes/SetBuildFlagsBuildCheck