spec: Open code %set_build_flags
    
    There should be no functional difference here, but it's not
    obvious at a glance how qemu handles globally defined CFLAGS + LDFLAGS
    with --extra-cflags and --extra-ldflags.
    
    Reproduce the desired behavior with explicit configure options and
    RPM variables
    
    Signed-off-by: Cole Robinson <crobinso@redhat.com>