Upgrade to snappy-1.1.9 & rewrite build_deps.sh as a Makefile

[hmmr]

In this PR, after attempts to get eleveldb to build on various arm arches, with mixed success:

• snappy is upgraded to latest (1.1.9), and
• c_src/build_deps.sh is rewritten as a proper Makefile;
• bump leveldb to the version proposed in this PR. Once the latter PR is reviewed, leveldb URL will need to be changed to basho repo.