fix build without gflags
fix build without gflags

Test Plan:
Built and ran with gflags:
% ./db_bench
LevelDB:    version 4.5
Date:       Tue Feb 16 12:04:23 2016
CPU:        40 * Intel(R) Xeon(R) CPU E5-2660 v2 @ 2.20GHz
...

And without gflags:
% ./db_bench
Please install gflags to run rocksdb tools
%

Reviewers: sdong, igor

Reviewed By: igor

Subscribers: igor, dhruba

Differential Revision: https://reviews.facebook.net/D54243

OverlordQ/rocksdb,wenduo/rocksdb,bbiao/rocksdb,norton/rocksdb,facebook/rocksdb,vmx/rocksdb,jalexanderqed/rocksdb,tsheasha/rocksdb,hobinyoon/rocksdb,wat-ze-hex/rocksdb,ryneli/rocksdb,bbiao/rocksdb,flabby/rocksdb,hobinyoon/rocksdb,tsheasha/rocksdb,Andymic/rocksdb,jalexanderqed/rocksdb,OverlordQ/rocksdb,flabby/rocksdb,facebook/rocksdb,ryneli/rocksdb,hobinyoon/rocksdb,SunguckLee/RocksDB,OverlordQ/rocksdb,facebook/rocksdb,Andymic/rocksdb,norton/rocksdb,facebook/rocksdb,hobinyoon/rocksdb,tsheasha/rocksdb,bbiao/rocksdb,flabby/rocksdb,wenduo/rocksdb,jalexanderqed/rocksdb,facebook/rocksdb,bbiao/rocksdb,tsheasha/rocksdb,vmx/rocksdb,wenduo/rocksdb,SunguckLee/RocksDB,SunguckLee/RocksDB,wat-ze-hex/rocksdb,hobinyoon/rocksdb,bbiao/rocksdb,wenduo/rocksdb,norton/rocksdb,ryneli/rocksdb,Andymic/rocksdb,Andymic/rocksdb,ryneli/rocksdb,OverlordQ/rocksdb,wenduo/rocksdb,OverlordQ/rocksdb,bbiao/rocksdb,norton/rocksdb,norton/rocksdb,vmx/rocksdb,norton/rocksdb,SunguckLee/RocksDB,vmx/rocksdb,hobinyoon/rocksdb,norton/rocksdb,bbiao/rocksdb,ryneli/rocksdb,wenduo/rocksdb,wat-ze-hex/rocksdb,vmx/rocksdb,wenduo/rocksdb,wenduo/rocksdb,Andymic/rocksdb,bbiao/rocksdb,OverlordQ/rocksdb,wat-ze-hex/rocksdb,ryneli/rocksdb,ryneli/rocksdb,jalexanderqed/rocksdb,tsheasha/rocksdb,tsheasha/rocksdb,wat-ze-hex/rocksdb,wat-ze-hex/rocksdb,vmx/rocksdb,OverlordQ/rocksdb,jalexanderqed/rocksdb,jalexanderqed/rocksdb,Andymic/rocksdb,vmx/rocksdb,vmx/rocksdb,facebook/rocksdb,norton/rocksdb,hobinyoon/rocksdb,SunguckLee/RocksDB,tsheasha/rocksdb,facebook/rocksdb,flabby/rocksdb,SunguckLee/RocksDB,Andymic/rocksdb,wat-ze-hex/rocksdb,wat-ze-hex/rocksdb,flabby/rocksdb,SunguckLee/RocksDB,hobinyoon/rocksdb,flabby/rocksdb,SunguckLee/RocksDB,facebook/rocksdb,jalexanderqed/rocksdb,flabby/rocksdb,Andymic/rocksdb,tsheasha/rocksdb