setup.py: more proper examining if object callable
    
    Just for the sake of assurance that no case of callable is missed.
    
    See also:
    http://bugs.python.org/issue10518
    http://stackoverflow.com/questions/111234/what-is-a-callable-in-python
    
    Signed-off-by: Jan Pokorny <jpokorny@redhat.com>