Outer projects may already build this target, in which case we don't want to build it again.

Bug: b/148010928
Change-Id: I19eea95eb81f13495268f5109d5554ef16033598
Reviewed-on: https://swiftshader-review.googlesource.com/c/SwiftShader/+/40330Reviewed-by: Antonio Maiorano <amaiorano@google.com>
Tested-by: Ben Clayton <bclayton@google.com>
Kokoro-Presubmit: kokoro <noreply+kokoro@google.com>