Get SDL building and start on a vk_triangle sample using it.

Tested on Linux with CMake and Bazel.

Closes #125

PiperOrigin-RevId: 279750128
diff --git a/CMakeLists.txt b/CMakeLists.txt
index ac138f0..d5942cb 100644
--- a/CMakeLists.txt
+++ b/CMakeLists.txt
@@ -74,6 +74,10 @@
   add_subdirectory(build_tools/third_party/tensorflow/tensorflow/compiler/mlir/xla EXCLUDE_FROM_ALL)
 endif()
 
+if(${IREE_BUILD_DEBUGGER} OR ${IREE_BUILD_SAMPLES})
+  add_subdirectory(third_party/sdl2 EXCLUDE_FROM_ALL)
+endif()
+
 #-------------------------------------------------------------------------------
 # IREE top-level libraries
 #-------------------------------------------------------------------------------