For now, warn the user that we don't support building PowerVR examples
against SwiftShader on MacOS. We may eventually add support, see
b/161976310.

Bug: b/155827755
Bug: b/161976310
Change-Id: I52216664567f1c18f59dc302dd720128ed28fe7d
Reviewed-on: https://swiftshader-review.googlesource.com/c/SwiftShader/+/46988Reviewed-by: Nicolas Capens <nicolascapens@google.com>
Kokoro-Result: kokoro <noreply+kokoro@google.com>
Tested-by: Antonio Maiorano <amaiorano@google.com>