Compile dynamic library by default

Summary:
Per request, some users need to use dynamic rocksdb library instead of static one.

However currently the dynamic libraries have to be manually compiled by default, which is inconvenient. I made dymamic libraries to be compiled by default.

Test Plan: make clean; make; make clean;

Reviewers: haobo, sdong, dhruba, igor

Reviewed By: haobo

CC: leveldb

Differential Revision: https://reviews.facebook.net/D15117