Use a preprocessor flag to optionally depend on dynamic linking header

Summary:
we need to install libdl in order to
```
#include <dlfcn.h>
```

I'm turning this feature on based on host os. If user wants to enable it, they need to make sure `dlfcn.h` header is available which it should for most UNIX-like host OSes. Added comments for that.

Reviewed By: digantdesai, dbort

Differential Revision: D47886884

fbshipit-source-id: fdf4db841bd066dcf47f16ea622e56e35f48e660