Allow skipping creating C++ wrappers with cxxignore feature

This can be useful to avoid errors due to features not supported at C++
level in some tests.

Doc/Manual/C.html

@@ -706,7 +706,7 @@ Other ones are due to things that could be supported but haven't been implemente
 </ul>
 </p>
 
-<H3>36.6.1 Additional sections</H3>
+<H3>36.6.1 Additional customization possibilities</H3>
 
 Generated C++ code can be customized by inserting custom code in the following sections:
 
@@ -715,6 +715,11 @@ Generated C++ code can be customized by inserting custom code in the following s
   <li><tt>cxxcode</tt> for additional code to appear after the declarations of all wrapper classes, inside the module-specific namespace.</li>
 </ul>
 
+The following features are taken into account when generating C++ wrappers:
+<ul>
+  <li><tt>cxxignore</tt> May be set to skip generation of C++ wrappers for the given function or class, while still generating C wrappers for them.</li>
+</ul>
+
 <H3>36.6.2 Exception handling</H3>
 
 <p>

Source/Modules/c.cxx

@@ -828,6 +828,9 @@ public:
     except_check_start =
     except_check_end = "";
 
+    if (Checkattr(n, "feature:cxxignore", "1"))
+      return;
+
     // Usually generating wrappers for overloaded methods is fine, but sometimes their types can clash after applying typemaps and in this case we have no
     // choice but to avoid generating them, as otherwise we'd just generate uncompilable code.
     if (Getattr(n, "sym:overloaded")) {
@@ -980,6 +983,9 @@ public:
     if (!cxx_wrappers_.is_initialized())
       return;
 
+    if (Checkattr(n, "feature:cxxignore", "1"))
+      return;
+
     String* const classname = Getattr(n, "sym:name");
 
     scoped_dohptr base_classes(NewStringEmpty());