Compile all quickstarts on Windows+Bazel

[coryan]

At least on Windows, compiling each quickstart is too slow. The google-cloud-cpp dependencies, such as gRPC, are compiled for each quickstart. Even with GCS caching this takes a while. Not to mention the disk space requirements.

The point of these builds is to test the WORKSPACE files in each quickstart directory, and to verify the quickstarts still compile with the google-cloud-cpp release shown in the quickstart.

We may need a different setup, where all the quickstarts are compiled from a single WORKSPACE file, and somehow we verify that all WORKSPACE files are identical?

Until then, I think compiling a random subset is a good workaround.