added *Nix variable to support compiling on FreeBSD

With these changes I'm able to compile InfluxDB on FreeBSD with leveldb support.
Rocksdb and hyperleveldb are currently not supported, because there are no ports of current versions on freebsd. Since influxdb 0.8.4 the FreeBSD Port of rocksdb is outdated (3.2.1) and doesn't work anymore. As soon as the port gets updated I'll submit another PR to support rocksdb.