maint: correct quoting for gl_WARN_ADD
    
    3 out of 4 uses of gl_WARN_ADD() were incorrectly adding "" around
    the argument, which in turn resulted in the argument being used
    unquoted (configure had gl_positive=""-fstack-protector-all"",
    rather than the intended gl_positive="-fstack-protector-all").
    
    Signed-off-by: Eric Blake <eblake@redhat.com>
    Reviewed-by: Andrea Bolognani <abologna@redhat.com>