cuda/cl tags should be opt-in

[willscott]

having dealt with this for a bit now, i've decided i care more, and that the optional libraries need to be opt-in parts of the build.

editors / standard ecosystem tools expect a 'go build' without tags to succeed, and tags are not always available. (e.g. atom doesn't have a way to set them https://www.bountysource.com/issues/42210626-does-go-plus-support-build-tags)

If i can't run go build successfully in the project because it fails on cuda headers that i can't install / test because my computer doesn't support them, i'm just not going to run the tests.