change calling conventions of isl_basic_set_has_defining_{,in}equalit{y,ies}

change calling conventions of isl_basic_set_has_defining_{,in}equalit{y,ies}

Before, these functions would only allow detection of defining
(in)equalities on set dimensions.  Rather than providing new
functions for detection on other kinds of variables, we generalize
the existing implementation to handle all kinds of variables.