Add clang tidy to build.

This is only a first approxmiation of what I would like for CI. Ideally,
it'd be it's own step that can pass/fail regardless of the build status.
With this setup, it's integrated into the build (and also running tidy
over our vendored dependencies, which are outside our scope).

Something like this: http://www.stablecoder.ca/2018/10/30/full-cmake-helper-suite.html
might be better for the future...