Conditionally include stdlib.h, unistd.h.

(VFORK_RETURN_TYPE): Remove.

src/sysdep.c

@@ -22,6 +22,9 @@ Boston, MA 02111-1307, USA.  */
 #include <config.h>
 #include <signal.h>
 #include <setjmp.h>
+#ifdef HAVE_UNISTD_H
+#include <unistd.h>
+#endif
 
 #include "lisp.h"
 #include "blockinput.h"
@@ -75,6 +78,10 @@ extern int h_errno;
 #include <unistd.h>
 #endif
 
+#ifdef HAVE_STDLIB_H
+#include <stdlib.h>
+#endif
+
 #ifdef HAVE_SETPGID
 #if !defined (USG) || defined (BSD_PGRPS)
 #define setpgrp setpgid
@@ -205,10 +212,6 @@ struct utimbuf {
 #endif
 #endif
 
-#ifndef VFORK_RETURN_TYPE
-#define VFORK_RETURN_TYPE int
-#endif
-
 /* LPASS8 is new in 4.3, and makes cbreak mode provide all 8 bits.  */
 #ifndef LPASS8
 #define LPASS8 0