configure.py: define ASAN_ENABLED when appropriate
configure.py: define ASAN_ENABLED when appropriate

Both GCC and Clang have predefined macros when AddressSanitizer is
being used, but unfortunately they use different ones.
It is much simpler to just make configure.py define ASAN_ENABLED
when the sanitizer is enabled.

Signed-off-by: Paweł Dziepak <e29bb3265626c60caf76d587cabdfcd8b379ef80@scylladb.com>

avikivity/seastar,dreamsxin/seastar,scylladb/seastar,scylladb/seastar,kjniemi/seastar,kjniemi/seastar,syuu1228/seastar,avikivity/seastar,avikivity/seastar,scylladb/seastar,syuu1228/seastar,dreamsxin/seastar,cloudius-systems/seastar,dreamsxin/seastar,syuu1228/seastar,cloudius-systems/seastar,kjniemi/seastar,cloudius-systems/seastar