Added initial OpenCL stub.

src/3rdparty/CL/README.md

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+# OpenCL<sup>TM</sup> API Headers
+
+This repository contains C language headers for the OpenCL API.
+
+The authoritative public repository for these headers is located at:
+
+https://github.com/KhronosGroup/OpenCL-Headers
+
+Issues, proposed fixes for issues, and other suggested changes should be
+created using Github.
+
+## Branch Structure
+
+The OpenCL API headers in this repository are Unified headers and are designed
+to work with all released OpenCL versions.  This differs from previous OpenCL
+API headers, where version-specific API headers either existed in separate
+branches, or in separate folders in a branch.
+
+## Compiling for a Specific OpenCL Version
+
+By default, the OpenCL API headers in this repository are for the latest
+OpenCL version (currently OpenCL 2.2).  To use these API headers to target
+a different OpenCL version, an application may `#define` the preprocessor
+value `CL_TARGET_OPENCL_VERSION` before including the OpenCL API headers.
+The `CL_TARGET_OPENCL_VERSION` is a three digit decimal value representing
+the OpenCL API version.
+
+For example, to enforce usage of no more than the OpenCL 1.2 APIs, you may
+include the OpenCL API headers as follows:
+
+```
+#define CL_TARGET_OPENCL_VERSION 120
+#include <CL/opencl.h>
+```
+
+## Directory Structure
+
+```
+README.md               This file
+LICENSE                 Source license for the OpenCL API headers
+CL/                     Unified OpenCL API headers tree
+```
+
+## License
+
+See [LICENSE](LICENSE).
+
+---
+
+OpenCL and the OpenCL logo are trademarks of Apple Inc. used by permission by Khronos.