Detect ASan at run time instead of compile time.

This way ASan-specific behavior (disabled seccomp filters) can be
enabled in the case when libminijail built w/o ASan is linked to an
ASan executable.

Bug: 27980137
Bug: 30252254
Change-Id: I219365041c339b65eb41d21d9a7dcbff7664f8e6
1 file changed