Add build rules for the GSL

This provides many helpful things, such as span.

Change-Id: I4d2a88b0a7cb93fe36a98f21f4a6874e335e063e
diff --git a/third_party/GSL/BUILD b/third_party/GSL/BUILD
new file mode 100644
index 0000000..10fe510
--- /dev/null
+++ b/third_party/GSL/BUILD
@@ -0,0 +1,8 @@
+licenses(["notice"])
+
+cc_library(
+    name = "GSL",
+    hdrs = glob(["include/**"]),
+    includes = ["include"],
+    visibility = ["//visibility:public"],
+)