Allow building with perl 5.14.

Patch from Alex Hunsaker.

src/pl/plperl/plperl.c

@@ -685,7 +685,7 @@ plperl_trusted_init(void)
 		if (!isGV_with_GP(sv) || !GvCV(sv))
 			continue;
 		SvREFCNT_dec(GvCV(sv)); /* free the CV */
-		GvCV(sv) = NULL;		/* prevent call via GV */
+		GvCV_set(sv, NULL);		/* prevent call via GV */
 	}
 	hv_clear(stash);
 	/* invalidate assorted caches */

src/pl/plperl/plperl.h

@@ -43,6 +43,11 @@
 #undef bool
 #endif
 
+/* supply GvCV_set if it's missing - ppport.h doesn't supply it, unfortunately */
+#ifndef GvCV_set
+#define GvCV_set(gv, cv)		(GvCV(gv) = cv)
+#endif
+
 /* routines from spi_internal.c */
 int			spi_DEBUG(void);
 int			spi_LOG(void);