12705 remove __builtin_alloca() declaration from string.h

*** 20,29 ****
--- 20,30 ----
   */
  
  /*
   * Copyright 2014 Garrett D'Amore <garrett@damore.org>
   * Copyright (c) 1989, 2010, Oracle and/or its affiliates. All rights reserved.
+  * Copyright 2020 Joyent, Inc.
   */
  
  /*      Copyright (c) 1988 AT&T */
  /*        All Rights Reserved   */
  
*** 137,154 ****
  extern char *strdup(const char *);
  #endif
  
  #if defined(__EXTENSIONS__) || \
          (!defined(_STRICT_STDC) && !defined(__XOPEN_OR_POSIX))
  #if defined(__GNUC__)
  
- /*
-  * gcc provides this inlining facility but Studio C does not.
-  * We should use it exclusively once Studio C also provides it.
-  */
- extern void *__builtin_alloca(size_t);
- 
  #define strdupa(s)                                                      \
          (__extension__(                                                 \
          {                                                               \
          char *__str = (char *)(s);                                      \
          strcpy((char *)__builtin_alloca(strlen(__str) + 1), __str);     \
--- 138,150 ----
  extern char *strdup(const char *);
  #endif
  
  #if defined(__EXTENSIONS__) || \
          (!defined(_STRICT_STDC) && !defined(__XOPEN_OR_POSIX))
+ 
  #if defined(__GNUC__)
  
  #define strdupa(s)                                                      \
          (__extension__(                                                 \
          {                                                               \
          char *__str = (char *)(s);                                      \
          strcpy((char *)__builtin_alloca(strlen(__str) + 1), __str);     \