Use pkg-config to find system leveldb

[njwhite]

Don't embed our own copy of LevelDB - build against the system on (the one pkg-config knows about) instead.

[vjoel]

I've been down this road already :)

https://github.com/vjoel/ruby-leveldb-native

Would you like to join that repo?

Btw, @wmorgan no longer maintains this repo and gave his blessing to ruby-leveldb-native.

[vjoel]

Another approach, instead of pkg-config (not sure how widely installed that is outside of linux), would be to use rake-compiler, as in https://github.com/minad/lmdb. This repo is sort of the best of both worlds: it has a bundled version of lmdb, but tries to find installed libs first. (And you can override that too.)

[vjoel]

I've been meaning to use rake-compiler in https://github.com/vjoel/ruby-leveldb-native, but I'm just more interested in working on lmdb now.

[njwhite]

@vjoel I will, thanks!