Well, FreeBSD does it by calling a sh builtin:

  		/*
  		 * We are the child; make /bin/sh expand `words'.
  		 */
  		(void)__libc_sigprocmask(SIG_SETMASK, 
  &oldsigblock, NULL);
  		if ((pdes[1] != STDOUT_FILENO ?
  		    _dup2(pdes[1], STDOUT_FILENO) :
  		    _fcntl(pdes[1], F_SETFD, 0)) < 0)
  			_exit(1);
  		if (_fcntl(pdesw[0], F_SETFD, 0) < 0)
  			_exit(1);
  		execl(_PATH_BSHELL, "sh", flags & WRDE_UNDEF ? "-u" : "+u",
  		    "-c", "IFS=$1;eval \"$2\";"
  		    "freebsd_wordexp -f \"$3\" ${4:+\"$4\"}",
  		    "",
  		    ifs != NULL ? ifs : " \t\n",
  		    flags & WRDE_SHOWERR ? "" : "exec 2>/dev/null",
  		    wfdstr,
  		    flags & WRDE_NOCMD ? "-p" : "",
  		    (char *)NULL);
  		_exit(1);
    	}