configure.ac: Improve C99 compatibility of __progname check

The check uses printf, so it needs to include <stdio.h> for compilers
which do not support implicit function declarations. (They were
removed from C99.)

Submitted upstream:

  <https://gitlab.freedesktop.org/libbsd/libbsd/-/merge_requests/23>

diff -ur libbsd-0.11.7.orig/configure.ac libbsd-0.11.7/configure.ac
--- libbsd-0.11.7.orig/configure.ac	2022-10-06 04:06:31.000000000 +0200
+++ libbsd-0.11.7/configure.ac	2022-12-12 10:00:57.500065200 +0100
@@ -221,7 +221,9 @@
 
 AC_MSG_CHECKING([for __progname])
 AC_LINK_IFELSE(
-	[AC_LANG_PROGRAM([[extern char *__progname;]],
+	[AC_LANG_PROGRAM([[
+	#include <stdio.h>
+	extern char *__progname;]],
 	                 [[printf("%s", __progname);]])],
 	[AC_DEFINE([HAVE___PROGNAME], [1], [Define to 1 if you have __progname])
 	 AC_MSG_RESULT([yes])],