build(CppAD): brought in CppAD for autodiff

we need an autodiff library at some point (or we need to roll our own but I do not think that makes sense). CppAD is well tested and header only and easy to include. It is also Liscene compatible with GPL v3.0. Here we bring it in as a dependency

build-config/cppad/include/cppad/local/is_pod.hpp.in

@@ -0,0 +1,41 @@
+# ifndef CPPAD_LOCAL_IS_POD_HPP
+# define CPPAD_LOCAL_IS_POD_HPP
+/* --------------------------------------------------------------------------
+CppAD: C++ Algorithmic Differentiation: Copyright (C) 2003-20 Bradley M. Bell
+
+CppAD is distributed under the terms of the
+             Eclipse Public License Version 2.0.
+
+This Source Code may also be made available under the following
+Secondary License when the conditions for such availability set forth
+in the Eclipse Public License, Version 2.0 are satisfied:
+      GNU General Public License, Version 2.0 or later.
+---------------------------------------------------------------------------- */
+// make sure size_t is defined because autotools version of
+// is_pod_specialize_98 uses it
+# include <cstddef>
+
+/*!
+\file is_pod.hpp
+File that defines is_pod<Type>(void)
+*/
+namespace CppAD { namespace local { // BEGIN_CPPAD_LOCAL_NAMESPACE
+/*!
+Is this type plain old data; i.e., its constructor need not be called.
+
+The default definition is false. This include file defines it as true
+for all the fundamental types except for void and nullptr_t.
+*/
+template <class T> bool is_pod(void) { return false; }
+// The following command suppresses doxygen processing for the code below
+/// \cond
+// C++98 Fundamental types
+@is_pod_specialize_98@
+
+// C++11 Fundamental types
+@is_pod_specialize_11@
+
+/// \endcond
+} } // END_CPPAD_LOCAL_NAMESPACE
+
+# endif