Add wren.hpp.

src/include/wren.hpp

@@ -0,0 +1,11 @@
+#ifndef wren_hpp
+#define wren_hpp
+
+// This is a convenience header for users that want to compile Wren as C and
+// link to it from a C++ application.
+
+extern "C" {
+  #include "wren.h"
+}
+
+#endif