Revamp our build tools

Summary:
This diff is revamping our build tools:
1) Use third-party2 instead of third-party
2) consolidate clang and gcc scripts together, lots of duplication there
3) remove hdfs libs, we never compile rocksdb with them

clang compilation doesn't work yet. It doesn't work in master either. I plan to fix it soon, but I just spent 2 hours trying to make it work and failed. I'll ask experts.

Test Plan: compiles with gcc

Reviewers: ljin, yhchiang, rven, sdong

Reviewed By: sdong

Subscribers: dhruba, leveldb

Differential Revision: https://reviews.facebook.net/D28047