RT2308: Add extern "C" { ... } wrapper

Add the wrapper to all public header files (Configure
generates one).  Don't bother for those that are just
lists of #define's that do renaming.

Reviewed-by: Tim Hudson <tjh@openssl.org>

Cherry-pick of commit 17e80c6bd0

crypto/pqueue/pqueue.h

@@ -64,6 +64,9 @@
 #include <stdlib.h>
 #include <string.h>
 
+#ifdef  __cplusplus
+extern "C" {
+#endif
 typedef struct _pqueue *pqueue;
 
 typedef struct _pitem
@@ -91,4 +94,7 @@ pitem *pqueue_next(piterator *iter);
 void   pqueue_print(pqueue pq);
 int    pqueue_size(pqueue pq);
 
+#ifdef  __cplusplus
+}
+#endif
 #endif /* ! HEADER_PQUEUE_H */