Undo redefinition of unique_ptr at end of header files.

It seems that clang both defines __cplusplus < 201103L and provides
unique_ptr (GitHub issue #215).

pform_types.h

@@ -380,4 +380,8 @@ extern std::ostream& operator<< (std::ostream&out, const pform_name_t&);
 extern std::ostream& operator<< (std::ostream&out, const name_component_t&that);
 extern std::ostream& operator<< (std::ostream&out, const index_component_t&that);
 
+#if __cplusplus < 201103L
+#undef unique_ptr
+#endif
+
 #endif /* IVL_pform_types_H */

vhdlpp/expression.h

@@ -1097,4 +1097,8 @@ private:
     Expression*delay_;
 };
 
+#if __cplusplus < 201103L
+#undef unique_ptr
+#endif
+
 #endif /* IVL_expression_H */