The idea of having to go out and download something in order for your module to build always seems like a "bad idea" to me, especially for users that are installing your module onto systems that do not have internet access (like those of us living behind corporate firewalls)
I would try to include the latest entity definitions from w3 as of the version of the module you're releasing and provide the user the option to download the latest/greatest or provide documentation of how to manually go get it instead of making it part of the automated build process.