[nodes] Drop VariableRef.__cmp__ implementation (closes #1190458)
The existing implementation relies on hash returning different values
for objects that compared equal. This is horribly wrong. Instead, stop
implementing comparison, and use the is_equivalent method explicitly.
#include "gecode/support.hh"
#include <stdio.h>
int main() {
#ifndef GECODE_VERSION
printf("2.1.2\n");
#else
printf("%s\n", GECODE_VERSION);
#endif
}