only build isl.py and its dependencies when extract_interface is available

The isl.py target in itself is fairly harmless since isl.py
is not built by default and therefore only results in failures
when the bindings are being built explicitly (or on "make dist).
However, barvinok-0.40-18-gb2a61c464a (interface: make language bindings
depend on all relevant headers, Sun Feb 18 12:30:10 2018 +0100)
added a libdep.a target which does get built by default while
it can only be built with a bundled or build isl submodule.
Hide the entire isl.py machinery behind a conditional to avoid
failures when a system isl submodule is being used.

Reported-by: Marcin Copik <mcopik@gmail.com>
Signed-off-by: Sven Verdoolaege <sven.verdoolaege@gmail.com>