Build shared lib and put linker flags to the end

I was trying to use rocksdb as a shared lib and keep getting some unresolved symbols related to bz2.

As a solution, I changed the relevant Makefile section, and put LDFLAGS after the object files, and the problem gone. BTW, there was a similar fix in leveldb, too:

https://code.google.com/p/leveldb/source/detail?r=40768657bc8ec3ded60712eeeab7c25b1b07deca

And leveldb is building shared lib at default target, so I added that too.