Various fixes for GCC 15.

--- safecat-1.13/alloc.c	2000-02-29 04:10:12.000000000 +0000
+++ safecat-1.13/alloc.c	2025-07-04 05:34:46.785127264 +0100
@@ -1,7 +1,6 @@
 #include "alloc.h"
 #include "error.h"
-extern char *malloc();
-extern void free();
+#include <stdlib.h>
 
 #define ALIGNMENT 16 /* XXX: assuming that this alignment is enough */
 #define SPACE 4096 /* must be multiple of ALIGNMENT */
--- safecat-1.13/safecat.c	2004-08-17 02:14:34.000000000 +0100
+++ safecat-1.13/safecat.c	2025-07-04 05:32:13.116256151 +0100
@@ -23,6 +23,7 @@
 #include <errno.h>
 #include <fcntl.h>
 #include <signal.h>
+#include <stdlib.h>
 #include <unistd.h>
 
 /* Support for large files (on Linux systems, if the appropriate
--- safecat-1.13/trycpp.c	2000-02-29 04:10:12.000000000 +0000
+++ safecat-1.13/trycpp.c	2025-07-04 05:38:03.049779818 +0100
@@ -1,7 +1,7 @@
 void main()
 {
 #ifdef NeXT
-  printf("nextstep\n"); exit(0);
+  printf("nextstep\n"); return 0;
 #endif
-  printf("unknown\n"); exit(0);
+  printf("unknown\n"); return 0;
 }
--- safecat-1.13/tryerrno.c	2004-02-22 02:25:34.000000000 +0000
+++ safecat-1.13/tryerrno.c	2025-07-04 05:37:25.528537780 +0100
@@ -1,6 +1,6 @@
 #include <errno.h>
 
-void main() {
+int main() {
   errno = 0;
-  _exit(errno);
+  return errno;
 }
--- safecat-1.13/tryulong32.c	2000-02-29 04:10:12.000000000 +0000
+++ safecat-1.13/tryulong32.c	2025-07-04 05:38:22.105334436 +0100
@@ -6,6 +6,6 @@
  u += u; u += u; u += u; u += u; u += u; u += u; u += u; u += u;
  u += u; u += u; u += u; u += u; u += u; u += u; u += u; u += u;
  u += u; u += u; u += u; u += u; u += u; u += u; u += u; u += u;
- if (!u) _exit(0);
- _exit(1);
+ if (!u) return 0;
+ return 1;
 }
--- safecat-1.13/tryulong64.c	2002-05-27 03:26:01.000000000 +0100
+++ safecat-1.13/tryulong64.c	2025-07-04 05:38:33.886736594 +0100
@@ -6,6 +6,6 @@
   u += u; u += u; u += u; u += u; u += u; u += u; u += u; u += u;
   u += u; u += u; u += u; u += u; u += u; u += u; u += u; u += u;
   u += u; u += u; u += u; u += u; u += u; u += u; u += u; u += u;
-  if (!u) _exit(1);
-  _exit(0);
+  if (!u) return 1;
+  return 0;
 }