Portable build that support cpus older than Intel Haswell

[ailisp]

CPU older than Intel Haswell (e.g. sandy bridge and ivy bridge) doesn't have avx2. Currently rust-rocksdb doesn't provide an option to compile it without avx2. rocksdb itself has a PORTABLE option in cmake, so passing a PORTABLE flag to it will be make a portable build: https://github.com/facebook/rocksdb/blob/e3169e3ea8762d2f34880742106858a23c8dc8b7/CMakeLists.txt#L222

Reference: parity's fork has this: https://github.com/paritytech/rust-rocksdb/blob/6e099ebea51e5b5adb33405310504c8582649b34/rocksdb-sys/build.rs#L52

[vitvakatu]

Probably duplicates #327