Note glslang's build targets may already have been added by the PowerVR
SDK. In that case, we use their copy. This is not optimal since it
potentially makes our test results depend on whether or not building the
PowerVR samples has been enabled, but it prevents duplicate targets.

Bug: b/158655757
Change-Id: Ic4e9292da9b1bb498164929cc7f93e73bf1fd6e6
Reviewed-on: https://swiftshader-review.googlesource.com/c/SwiftShader/+/45628
Presubmit-Ready: Nicolas Capens <nicolascapens@google.com>
Tested-by: Nicolas Capens <nicolascapens@google.com>
Reviewed-by: Antonio Maiorano <amaiorano@google.com>
Kokoro-Result: kokoro <noreply+kokoro@google.com>