[Python] Improve handling of SWIG_Py*Method_New

The SWIG_PyInstanceMethod_New method is no longer added to wrapped
classes except when it's actually needed, which is when
(!builtin && fastproxy) is true (which it isn't by default).

The SWIG_PyStaticMethod_New method is no longer is now similarly
gated - previously only (fastproxy) was checked.

Finally the C/C++ functions that implement these were always compiled
into the module, but now they're only included if
(!builtin && fastproxy) is true.

Issue noted by vadz in #2190.

Lib/python/pyinit.swg

@@ -8,6 +8,8 @@
 %fragment("<stddef.h>"); // For offsetof
 #endif
 
+#if defined SWIGPYTHON_FASTPROXY && !defined SWIGPYTHON_BUILTIN
+
 %insert(runtime) %{
 #ifdef __cplusplus
 extern "C" {
@@ -24,6 +26,8 @@ SWIGINTERN PyObject *SWIG_PyStaticMethod_New(PyObject *SWIGUNUSEDPARM(self), PyO
 #endif
 %}
 
+#endif
+
 %init %{
 
 #ifdef __cplusplus
@@ -103,6 +107,8 @@ SWIG_Python_FixMethods(PyMethodDef *methods, const swig_const_info *const_table,
   }
 } 
 
+#if defined SWIGPYTHON_FASTPROXY && !defined SWIGPYTHON_BUILTIN
+
 /* -----------------------------------------------------------------------------
  * Method creation and docstring support functions
  * ----------------------------------------------------------------------------- */
@@ -161,6 +167,8 @@ SWIGINTERN PyObject *SWIG_PyStaticMethod_New(PyObject *SWIGUNUSEDPARM(self), PyO
   return PyStaticMethod_New(func);
 }
 
+#endif
+
 #ifdef __cplusplus
 }
 #endif