initial import of isl 0.26.

this enables additional optimisations in GCC. from the README:

isl is a thread-safe C library for manipulating sets and relations
of integer points bounded by affine constraints. The descriptions of
the sets and relations may involve both parameters and existentially
quantified variables. All computations are performed in exact integer
arithmetic using GMP.

isl is released under the MIT license, but depends on the LGPL GMP
library.