Implement support in the build system for annotating symbols for export, as with _declspec(dllexport) or attribute_((visibility("default"))), and switch the build on non-Windows platforms to build with the default visibility set to hidden.

We can simplify the mechanics of this task by leveraging a preprocesor technique outlined here:

http://stackoverflow.com/questions/40944257/can-boost-pp-defined-be-implemented