Better handle fedabipkgdiff dependencies detection
    
    Even when the configure script is executed without the
    --enable-fedabipkgdiff switch, if one of the dependencies of
    fedabipkgdiff is missing (e.g, python), it fails with a fatal error.
    
    It should rather just disable fedabipkgdiff in that case.
    
    This patch fixes this behaviour.  It makes it so that it's only when
    the --enable-fedabipkgdiff switch provided that a missing dependency
    results in a fatal hard error.
    
    	* configure.ac: It's only when --enable-fedabipkgdiff is provided
    	that a missing dependency of the fedabipkgdiff program results in
    	a fatal error.  Otherwise, building fedabipkgdiff is just
    	disabled.
    
    Signed-off-by: Dodji Seketeli <dodji@redhat.com>