core: main: fix CLooG check for Ubuntu's insanity

In Ubuntu's grand quest to inhibit linux standardization,
they renamed the cloog binary to cloog-isl, and didn't leave
a symlink for compatibility with the real package. This commit
works around their needless forking/modification.

Off topic:
In regards to the cloog package, ubuntu also turned one package/tarball
into three separate packages with no source alternative (in debian,
installation is possible with the cloog package alone).