A more C++-friendly dNOOP.

The problem with Perl___notused under C++ is that in some cases
it's merely extern, and in some cases (via the XS macro via the
XSPROTO macro) it's extern "C".  Object code analysis shows that
you do actually get one mangled and one unmangled version of the
symbol, which wouldn't matter since the whole point is to have
something we never use.

Except that one very picky C++ compiler (HP C++ for OpenVMS) sees
what we're up to and slaps us down hard.  Since declaration after
statement has always been allowed in C++, just go ahead and do a
real noop statement for C++ and avoid the use of an external
symbol.

perl.h

@@ -359,7 +359,11 @@
 /* Rats: if dTHR is just blank then the subsequent ";" throws an error */
 /* Declaring a *function*, instead of a variable, ensures that we don't rely
    on being able to suppress "unused" warnings.  */
+#ifdef __cplusplus
+#define dNOOP (void)0
+#else
 #define dNOOP extern int Perl___notused(void)
+#endif
 
 #ifndef pTHX
 /* Don't bother defining tTHX and sTHX; using them outside