Fix CMake failure on MacOS when PowerVR submodule is checked out

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/+/46988
Reviewed-by: Nicolas Capens <nicolascapens@google.com>
Kokoro-Result: kokoro <noreply+kokoro@google.com>
Tested-by: Antonio Maiorano <amaiorano@google.com>

CMakeLists.txt

diff --git a/CMakeLists.txt b/CMakeLists.txt
index 65fafb4..af07cf7 100644
--- a/CMakeLists.txt
+++ b/CMakeLists.txt
@@ -732,6 +732,12 @@
 # Sample programs and tests
 ###########################################################
 
+# TODO(b/161976310): Add support for building PowerVR on MacOS
+if(APPLE AND HAVE_PVR_SUBMODULE AND SWIFTSHADER_BUILD_PVR)
+    message(WARNING "Building PowerVR examples for SwiftShader is not yet supported on Apple platforms.")
+    set(SWIFTSHADER_BUILD_PVR FALSE)
+endif()
+
 if(HAVE_PVR_SUBMODULE AND SWIFTSHADER_BUILD_PVR)
     if(UNIX AND NOT APPLE)
         set(PVR_WINDOW_SYSTEM XCB)