Add support for INTERFACE_ENABLE_EXPORTS
We have a library that will not work properly unless executables that link to it have set ENABLE_EXPORTS. Currently, we have to tell our users to do this, or we have to set CMAKE_ENABLE_EXPORTS, which is a bit of heavy hammer.
It would be nice for there to be an INTERFACE_ENABLE_EXPORTS so that this property could be inherited by dependencies in a more principled way.