link against isl libraries first

link against isl libraries first

In particular, link against the isl libraries before linking
against libLLVM.  If LLVM was compiled with Polly support,
then it will also contain isl symbols.  This is usually not a problem
since these symbols are versioned.  However, if libLLVM is
a shared library and libpet is dynamically loaded,
then libpet may end up calling isl symbols inside libLLVM.
Linking against libisl first prevent this.

Signed-off-by: Sven Verdoolaege <sven.verdoolaege@gmail.com>