Use %set_build_flags macro to set CFLAGS/CXXFLAGS/LDFLAGS
    
    We need to call this manually as the build doesn't use any of the
    standard %configure or %cmake or %meson macros that call
    %set_build_flags.
    
    Previously, the build was setting CFLAGS and CXXFLAGS, but not LDFLAGS,
    which meant hardening flags weren't correctly passed to the linker and
    flatpak builds didn't get -L/app/lib correctly set.
    
    Calling %set_build_flags fixes both.