Fix -Wimplicit-function-declaration errors with gcc 14.

Set `STDC_HEADERS` to include `stdio.h` and `stdlib.h`.

```
expr_parser.c: In function 'pamtester_expr_parser_hdlr':
expr_parser.c:87:31: error: implicit declaration of function 'strtol'; did you mean 'strtok'? [-Wimplicit-function-declaration]
   87 |                         val = strtol(token_val, &next_ptr, 10);
      |                               ^~~~~~
      |                               strtok
```

```
parse_opts.c: In function 'pamtester_parse_opts':
parse_opts.c:85:81: error: implicit declaration of function 'sprintf' [-Wimplicit-function-declaration]
   85 |                         sprintf(*err_msg, "Option \"%s\" may not be specified twice or more", opt->long_name);
      |                         ^~~~~~~
parse_opts.c:51:1: note: include '<stdio.h>' or provide a declaration of 'sprintf'
   50 | #include "parse_opts.h"
  +++ |+#include <stdio.h>
```

--- pamtester-0.1.2-origin/configure
+++ pamtester-0.1.2/configure
@@ -20432,7 +20432,7 @@
 }'
 fi
 
-DEFS=-DHAVE_CONFIG_H
+DEFS="-DHAVE_CONFIG_H -DSTDC_HEADERS"
 
 ac_libobjs=
 ac_ltlibobjs=