Much quicker at incremental builds.
Also print when the build has finished.

Change-Id: I3bd74ddb501b4a2584fc2cbd9bd85b250ce117be
Reviewed-on: https://swiftshader-review.googlesource.com/c/SwiftShader/+/31834Reviewed-by: Antonio Maiorano <amaiorano@google.com>
Reviewed-by: Nicolas Capens <nicolascapens@google.com>
Tested-by: Ben Clayton <bclayton@google.com>
Kokoro-Presubmit: kokoro <noreply+kokoro@google.com>