add shared library support to add_fbthrift_cpp_library
add shared library support to add_fbthrift_cpp_library

Summary:
`add_fbthrift_cpp_library` should honor `BUILD_SHARED_LIBS`, and call
`add_library` with the right setting (`SHARED` if enabled, `STATIC` otherwise)

Reviewed By: yns88

Differential Revision: D24911124

fbshipit-source-id: 79df7640a758a592a3df3e9e79bb129dd57f2d47

facebook/wangle,facebook/wangle,facebook/wangle