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.
Loop execution time reporting

Set clast_for->time_var_name to the name of the variable which will
accumulate execution time of the body inside this loop (in seconds
double precision floating-point) if macro TIME is defined (-DTIME).
The code is linked to cloog_util_rtclock() that relies on gettimeofday
and is part of the CLooG library.

Signed-off-by: Uday Bondhugula <udayreddy@gmail.com>
Signed-off-by: Cedric Bastoul <cedric.bastoul@u-psud.fr>