build-sys: add ./configure --enable-address-sanitizer

Enabling address sanitizer seems like a useful thing, but is quite
tricky. Proper flags have to be passed to CPPFLAGS, CFLAGS and
LDFLAGS, but passing them on the commandline doesn't work because
we tests are done with ld directly, and not with libtool like in
real linking. We might want to fix this, but let's add a handy
way to enable address checking anyway.