Enable building shared library
Enable building shared library

Summary:
The only questionable part of these changes is removing `$<$<NOT:$<CXX_COMPILER_ID:Clang>>:-latomic>)`. Feel free to drop that hunk.
Closes https://github.com/facebook/wangle/pull/58

Differential Revision: D3856442

Pulled By: djwatson

fbshipit-source-id: 42c1aec53c4acce5a36b7c53d0a575c8802ba55b

facebook/wangle,facebook/wangle,facebook/wangle