fix error on -Wimplicit-function-declaration in configure & compile

> conftest.c:50:14: error: call to undeclared library function 'exit' with type 'void (int) __attribute__((noreturn))'; ISO C99 and later do not support implicit function declarations [-Wimplicit-function-declaration]
> conftest.c:102:2: error: call to undeclared function 'wait'; ISO C99 and later do not support implicit function declarations [-Wimplicit-function-declaration]
> string.c:45:9: error: call to undeclared library function 'malloc' with type 'void *(unsigned long)'; ISO C99 and later do not support implicit function declarations [-Wimplicit-function-declaration]

diff '--color=auto' -urN a/configure.ac b/configure.ac
--- a/configure.ac	2004-10-18 16:09:36.000000000 -0000
+++ b/configure.ac	2025-02-26 18:23:10.470314477 -0000
@@ -63,6 +63,7 @@
 AC_TRY_RUN([
 #include <stdio.h>
 #include <signal.h>
+#include <stdlib.h>
 int main() { exit(SIGRTMIN >= 0 ? 0 : 1); }
 ], [
 AC_MSG_RESULT([yes])
@@ -94,6 +95,8 @@
 #include <fcntl.h>
 #include <signal.h>
 #include <unistd.h>
+#include <stdlib.h>
+#include <sys/wait.h>
 
 volatile int count = 0;
 
diff '--color=auto' -urN a/src/string.c b/src/string.c
--- a/src/string.c	2004-10-06 05:13:30.000000000 -0000
+++ b/src/string.c	2025-02-26 18:23:35.692315718 -0000
@@ -22,6 +22,7 @@
 #include <stdint.h>
 #include <limits.h>
 #include <errno.h>
+#include <stdlib.h>
 #include "dnotify.h"
 #include "gettext.h"
 #define _(s) gettext(s)