fix `Undefined symbols` without `HAVE_ZLIB`

In current version of this library, avoid zlib could not pass the ld. Error trace:
```
Undefined symbols for architecture x86_64:
  "_mailstream_compress_driver", referenced from:
      _mailstream_low_wait_idle in libetpan.a(mailstream_low.o)
  "_mailstream_low_compress_wait_idle", referenced from:
      _mailstream_low_wait_idle in libetpan.a(mailstream_low.o)
ld: symbol(s) not found for architecture x86_64
```