Dramatically speeds up builds when switching build flags.

Change-Id: I661d18458db69f6663381c252b06b3ed96f3a5c6
Reviewed-on: https://swiftshader-review.googlesource.com/c/SwiftShader/+/33472Tested-by: Ben Clayton <bclayton@google.com>
Reviewed-by: Nicolas Capens <nicolascapens@google.com>
Kokoro-Presubmit: kokoro <noreply+kokoro@google.com>