Fixes for GCC 15.

--- jam-2.5/execunix.c	2003-04-23 05:45:50.000000000 +0100
+++ jam-2.5/execunix.c	2025-07-04 16:47:01.712650786 +0100
@@ -102,7 +102,7 @@
 {
 	int pid;
 	int slot;
-	const char *argv[ MAXARGC + 1 ];	/* +1 for NULL */
+	char *argv[ MAXARGC + 1 ];	/* +1 for NULL */
 
 # ifdef USE_EXECNT
 	char *p;
--- jam-2.5/jam.c	2003-04-23 05:45:51.000000000 +0100
+++ jam-2.5/jam.c	2025-07-04 16:47:08.149551531 +0100
@@ -166,6 +166,7 @@
 # endif
 # endif
 
+int
 main( int argc, char **argv, char **arg_environ )
 {
 	int		n;
--- jam-2.5/jam.h	2004-06-23 18:45:36.000000000 +0100
+++ jam-2.5/jam.h	2025-07-04 16:46:38.338326515 +0100
@@ -363,6 +363,7 @@
 
 # include <sys/types.h>
 # include <sys/stat.h>
+# include <sys/wait.h>
 
 # ifndef OS_MPEIX
 # include <sys/file.h>
@@ -374,6 +375,7 @@
 # include <signal.h>
 # include <string.h>
 # include <time.h>
+# include <unistd.h>
 
 # ifndef OS_QNX
 # include <memory.h>
--- jam-2.5/mkjambase.c	2003-04-23 05:45:51.000000000 +0100
+++ jam-2.5/mkjambase.c	2025-07-04 16:47:22.337038510 +0100
@@ -24,6 +24,7 @@
 # include <stdio.h>
 # include <string.h>
 
+int
 main( int argc, char **argv, char **envp )
 {
 	char buf[ 1024 ];