Build: support minimal link flags

[yihuang]

Removing these flags seems have no side effects, but make life easier with my package manager (nix), where I only need to install librocksdb, rather than installing all the indirect dependencies, those are indirectly brought in anyway.

In many cases, we don't need the extra link flags, either because librocksdb already referenced the indirect dependencies by path, or it's not built with it at all, so this PR introduce a build tag grocksdb_clean_link to only link minimal dependencies.

[linxGnu]

Hello @yihuang , I am afraid of many current builds does not contains linking to lz4, snappy. Removing links might break those systems.

[yihuang]

Hello @yihuang , I am afraid of many current builds does not contains linking to lz4, snappy. Removing links might break those systems.

are those libs are built with librocksdb itself? if librocksdb is not built with them, it doesn't matter if we specify them here?

[linxGnu]

@yihuang yes, fyi, they are custom builds. There are many custom-builds like this.

[yihuang]

@yihuang yes, fyi, they are custom builds. There are many custom-builds like this.

I mean if the librocksdb is built with them, it should still doesn't matter if we specify here or not, do you get a link error if we don't specify the flags here?
We've been successfully using gorocksdb without these flags.

[yihuang]

Did you do static link?

[yihuang]

@linxGnu how about make it optional, I added a build tag grocksdb_clean_link, so the default behavior is not changed.

[linxGnu]

Sound great to me when using directive building. Let me have a look this week and feedback to you on Monday. Thank you for the contribution