Snappy support is broken

[benjoe87]

Snappy support is broken since 6ef9202.

The root cause seems like:

1. The build flags are not exported in the Makefile
2. The leveldb build cannot detect the snappy support and don't set the SNAPPY C marco https://github.com/basho/leveldb/blob/develop/build_detect_platform#L169
3. The leveldb part is builded without snappy support.

It is hard to detect this issue during the tests because leveldb simply do not compress the blocks if snappy support is not present.
https://github.com/basho/leveldb/blob/b91d5cee860bc708c10c48911586fdb347558ea3/table/table_builder.cc#L212

But it definitely breaks the upgrades mentioned in #273

Exporting these variables probably solves the issue but I am not 100% sure that this is a comprehensive solution.