Bug: hiredis driver conflict with the hiredis gem

[casperisfine]

I noticed this when working on the semian adapter.

Repro (linux only):

require 'redis'
require 'redis-client'

Redis.new(host: "redis", driver: :hiredis).ping
RedisClient.new(host: "redis", driver: :hiredis).call("ping")

From what I gather, we end up with redisContext->fd = 0, which later causes Errno::ENOTSOCK

[casperisfine]

A bit more info, while I can't explain the how and why, what seem to happen based on my various debug attempts, is that both extensions end up using the same libhiredis, e.g. if hiredis-rb is loaded first, the redis-client binding end up using the libhiredis compiled by hiredis-rb.

So it seems to be some kind of linking issue.

[casperisfine]

@flavorjones helpfully pointed me to https://github.com/sparklemotion/nokogiri/blob/1552b2aedf1f890016934731b239332c74c6f9a2/ext/nokogiri/extconf.rb#L966

$libs should allow to import .a inside the .so without exporting symbols.