The rule in Makefile.am generating isl.py redirects the output of a
subshell invoking multiple commands directly to the target file. This
causes isl.py to be created even if these commands fail.

Remove isl.py if the generating commands fail.

Signed-off-by: Andi Drebes <andi@drebesium.org>
Signed-off-by: Sven Verdoolaege <sven.verdoolaege@gmail.com>