tell bundled isl to build extract_interface
tell bundled isl to build extract_interface

This is needed to construct a copy of isl.py inside pet.
isl does not build extract_interface by default
because it requires the presence of clang.  Inside ppcg, clang is
required anyway, so simply tell isl that clang is available.

Signed-off-by: Sven Verdoolaege <e5350bbed4977f5eb8ae1dc6abd9ae59d21ace75@kotnet.org>

nicolasvasilache/ppcg,Meinersbur/ppcg,Meinersbur/ppcg,nicolasvasilache/ppcg