Fix ODR violations

Detected by compiling with GCC flags `-flto -Wodr`.

Source/Modules/overload.cxx

@@ -21,6 +21,7 @@
 String *argv_template_string;
 String *argc_template_string;
 
+namespace {
 struct Overloaded {
   Node *n;			/* Node                               */
   int argc;			/* Argument count                     */
@@ -28,6 +29,7 @@ struct Overloaded {
   int error;			/* Ambiguity error                    */
   bool implicitconv_function;	/* For ordering implicitconv functions*/
 };
+}
 
 static int fast_dispatch_mode = 0;
 static int cast_dispatch_mode = 0;

Source/Modules/python.cxx

@@ -93,6 +93,7 @@ static int nortti = 0;
 static int relativeimport = 0;
 
 /* flags for the make_autodoc function */
+namespace {
 enum autodoc_t {
   AUTODOC_CLASS,
   AUTODOC_CTOR,
@@ -103,7 +104,7 @@ enum autodoc_t {
   AUTODOC_CONST,
   AUTODOC_VAR
 };
-
+}
 
 static const char *usage1 = "\
 Python Options (available with -python)\n\

Source/Modules/r.cxx

@@ -1301,13 +1301,14 @@ void R::addAccessor(String *memberName, Wrapper *wrapper, String *name,
 
 #define MAX_OVERLOAD 256
 
+namespace {
 struct Overloaded {
   Node      *n;          /* Node                               */
   int        argc;       /* Argument count                     */
   ParmList  *parms;      /* Parameters used for overload check */
   int        error;      /* Ambiguity error                    */
 };
-
+}
 
 List * R::Swig_overload_rank(Node *n,
 	                     bool script_lang_wrapping) {

Source/Modules/ruby.cxx

@@ -116,6 +116,7 @@ public:
 
 
 /* flags for the make_autodoc function */
+namespace {
 enum autodoc_t {
   AUTODOC_CLASS,
   AUTODOC_CTOR,
@@ -127,6 +128,7 @@ enum autodoc_t {
   AUTODOC_SETTER,
   AUTODOC_NONE
 };
+}
 
 static const char *usage = "\
 Ruby Options (available with -ruby)\n\