libunwind required, but not available

libunwind appears to be a "hard" dependency, without a meson toggle to turn it on or off, or like auto-detect it.

As you can see in https://buildd.debian.org/status/package.php?p=libunwind libunwind is not available on a few architectures, which geary used to build on (ie. s390x alpha, and so on) https://buildd.debian.org/status/package.php?p=geary&suite=sid

It might be nice to make libunwind optional. Alternatively, if libunwind is really required, then we'd need to drop architectures in debian/ubuntu.