Two fixes for C++ compilation.

Explicit casts in guaranteed_memset and external declare low_hashmem
when it is a variable to avoid having multiple instances of it.

src/pike_memory.h

@@ -136,9 +136,9 @@ struct mem_searcher
  * sensitive data has to be cleared from memory.
  */
 static INLINE void * guaranteed_memset(void * p, int c, size_t n) {
-    volatile char * _p = p;
+    volatile char * _p = (char *)p;
     while (n--) *_p++ = c;
-    return p;
+    return (void *)p;
 }
 
 #include "pike_search.h"
@@ -159,6 +159,7 @@ PMOD_EXPORT void reverse(char *memory, size_t nitems, size_t size);
 PMOD_EXPORT void reorder(char *memory, INT32 nitems, INT32 size,INT32 *order);
 
 #if (defined(__i386__) || defined(__amd64__)) && defined(__GNUC__)
+PMOD_EXPORT
 #ifdef __i386__
 __attribute__((fastcall)) 
 #endif