Update isl to include isl_val changes

The latest version of isl includes a new data type isl_val, which properly hides
the multi precision math library used by isl. In Polly we would like to replace
all uses of isl_int with the corresponding isl_val interfaces. This will allow
us to switch the multi precision math library in isl. This is especially
interesting for people who would like to replace libgmp with a non-gpl licensed
library (e.g. imath).