core: add safe guard to avoid double specialization

Author: jcarpent

include/pycppad/fwd.hpp

@@ -14,6 +14,8 @@
 #include <cppad/cppad.hpp>
 #include <Eigen/Core>
 
+#ifndef PYCPPAD_EXCLUDE_EIGEN_NUMTRAITS_SPECIALIZATION
+
 namespace Eigen
 {
   template <class Base> struct NumTraits< CppAD::AD<Base> >
@@ -67,4 +69,6 @@ namespace Eigen
   };
 } // namespace Eigen
 
+#endif // ifndef PYCPPAD_EXCLUDE_EIGEN_NUMTRAITS_SPECIALIZATION
+
 #endif // ifndef __pycppad_fwd_hpp__