Deprecate jxcore functions

[jasonkarns]

JXcore has been abandoned. The builds no longer work and are to be removed ( #175 ). The build functions shouldn't be removed abruptly, as there may be people still relying on them, even with custom build definitions.

First step is to add warning messages to all the to-be-deprecated functions. That would be released as a minor version.

Then in the next major version bump, the functions could be removed entirely.