add GSL as a development dependency

external/include/gsl/gsl

@@ -0,0 +1,29 @@
+///////////////////////////////////////////////////////////////////////////////
+//
+// Copyright (c) 2015 Microsoft Corporation. All rights reserved.
+//
+// This code is licensed under the MIT License (MIT).
+//
+// THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR
+// IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY,
+// FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE
+// AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER
+// LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM,
+// OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN
+// THE SOFTWARE.
+//
+///////////////////////////////////////////////////////////////////////////////
+
+#ifndef GSL_GSL_H
+#define GSL_GSL_H
+
+#include <gsl/gsl_algorithm> // copy
+#include <gsl/gsl_assert>    // Ensures/Expects
+#include <gsl/gsl_byte>      // byte
+#include <gsl/gsl_util>      // finally()/narrow()/narrow_cast()...
+#include <gsl/multi_span>    // multi_span, strided_span...
+#include <gsl/pointers>      // owner, not_null
+#include <gsl/span>          // span
+#include <gsl/string_span>   // zstring, string_span, zstring_builder...
+
+#endif // GSL_GSL_H