Hidden behind a env variable.
Defines: * XRT_HAVE_[name] is a external dep indicator. * XRT_DRIVER_[name] if a internal driver is built. CMake variables: * BUILD_WITH_[name] is a external dep indicator. * BUILD_DRIVER_[name] tells if we should build a specific driver.
We had BUILD_WITH_OPENHMD but it was unused, while the HDK driver always built if the deps were found.