With cross-platform builds such as Envoy (https://github.com/lyft/envoy/), we would benefit from the ability to condition the inclusion of different dependencies and tests via either some form of platform tag or something like select but usable inside macros.

Today, we make use of select (https://github.com/lyft/envoy/blob/master/bazel/envoy_build_system.bzl#L35), but this is not flexible enough to indicate "this test should only be applied on Linux, not OS X", see envoyproxy/envoy#1365 (comment). If Bazel natively supported a tag such as "linux_only" or the like, this would make life easier.

@zuercher