Compile -O2 by default and add `make dbg`
Compile -O2 by default and add `make dbg`

Summary: To speed up the compilation while allowing us to compile in debug mode.

Test Plan:
make: see -O2 enabled
make dbg: didn't see -O2

Reviewers: igor

Reviewed By: igor

CC: leveldb, dhruba

Differential Revision: https://reviews.facebook.net/D15969

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hao0907/rocksdb,wenduo/rocksdb,tsheasha/rocksdb,kaschaeffer/rocksdb,Andymic/rocksdb,RyanTech/rocksdb,biddyweb/rocksdb,norton/rocksdb,Vaisman/rocksdb,anagav/rocksdb,wlqGit/rocksdb,virtdb/rocksdb,NickCis/rocksdb,RyanTech/rocksdb,SunguckLee/RocksDB,jalexanderqed/rocksdb,caijieming-baidu/rocksdb,biddyweb/rocksdb,NickCis/rocksdb,tschottdorf/rocksdb,wskplho/rocksdb,OverlordQ/rocksdb,wskplho/rocksdb,wlqGit/rocksdb,amyvmiwei/rocksdb,IMCG/RcoksDB,skunkwerks/rocksdb,vmx/rocksdb,SunguckLee/RocksDB,Andymic/rocksdb,virtdb/rocksdb,sorphi/rocksdb,anagav/rocksdb,vashstorm/rocksdb,RyanTech/rocksdb,wat-ze-hex/rocksdb,virtdb/rocksdb,IMCG/RcoksDB,facebook/rocksdb,virtdb/rocksdb,Vaisman/rocksdb,hobinyoon/rocksdb,ryneli/rocksdb,wlqGit/rocksdb,wskplho/rocksdb,Andymic/rocksdb,lgscofield/rocksdb,JoeWoo/rocksdb,vmx/rocksdb,dkorolev/rocksdb,alihalabyah/rocksdb,zhangpng/rocksdb,sorphi/rocksdb,norton/rocksdb,wenduo/rocksdb,ylong/rocksdb,amyvmiwei/rocksdb,wlqGit/rocksdb,dkorolev/rocksdb,bbiao/rocksdb,temicai/rocksdb,virtdb/rocksdb,jalexanderqed/rocksdb,siddhartharay007/rocksdb,IMCG/RcoksDB,facebook/rocksdb,JoeWoo/rocksdb,mbarbon/rocksdb,OverlordQ/rocksdb,norton/rocksdb,geraldoandradee/rocksdb,rDSN-Projects/rocksdb.replicated,NickCis/rocksdb,OverlordQ/rocksdb,tsheasha/rocksdb,lgscofield/rocksdb,geraldoandradee/rocksdb