Add a test_regenerate target for regenerating failing tests

"make test_regenerate" will update all failing tests with the
files generated by the current CLooG. Handy to update tests
that changed textually but remain semantically equivalent,
e.g., because of a polyhedral library update. Use with care!
Generate safety parentheses around compound expressions

Now CLooG generates statements with parentheses around expressions
with more than one term, e.g., S1(0,c1,(c2+c3)) instead of S1(0,c1,c2+c3).
The purpose of this change is to help users to avoid bugs when relying
on macros.