on ia64 architecture libunwind comes with gcc. Unfortunately
libunwind is not directly usable as-is and fails at link time:

```
    ia64-unknown-linux-gnu-g++ -o perf/.libs/local_lat perf/local_lat.o src/.libs/libzmq.so -lsodium -lrt -lpthread -ldl
    src/.libs/libzmq.so: undefined reference to `_ULia64_step'
```

The change adds --{enable,disable}-libunwind flag to control
automatic dependency. The default is unchanged: use if available.
Signed-off-by: Sergei Trofimovich <slyfox@gentoo.org>