Avoid implicit function declarations in the configure script.
The undeclared exit function comes from autoconf-generated code.
The SHUT_RD and h_errno checks are specific to the spamassassin
package.

Submitted upstream: <https://bz.apache.org/SpamAssassin/show_bug.cgi?id=8076>

diff --git a/spamc/configure b/spamc/configure
index d8e5dcf8a21d6a3d..5472d2b009fb9083 100755
--- a/spamc/configure
+++ b/spamc/configure
@@ -2624,8 +2624,8 @@ main ()
   for (i = 0; i < 256; i++)
     if (XOR (islower (i), ISLOWER (i))
 	|| toupper (i) != TOUPPER (i))
-      exit(2);
-  exit (0);
+      return 2;
+  return 0;
 }
 _ACEOF
 rm -f conftest$ac_exeext
@@ -3553,6 +3553,7 @@ _ACEOF
 cat confdefs.h >>conftest.$ac_ext
 cat >>conftest.$ac_ext <<_ACEOF
 /* end confdefs.h.  */
+#include <stdio.h>
 #include <sys/types.h>
 #include <sys/socket.h>
 int
@@ -4252,6 +4253,7 @@ _ACEOF
 cat confdefs.h >>conftest.$ac_ext
 cat >>conftest.$ac_ext <<_ACEOF
 /* end confdefs.h.  */
+#include <stdio.h>
 #include <netdb.h>
 int
 main ()
diff --git a/spamc/configure.in b/spamc/configure.in
index 42cc998412075086..96769105ef640bc8 100644
--- a/spamc/configure.in
+++ b/spamc/configure.in
@@ -47,7 +47,8 @@ dnl ----------------------------------------------------------------------
 
 AC_CACHE_CHECK([for SHUT_RD],
        shutrd, [
-                AC_TRY_COMPILE([#include <sys/types.h>
+                AC_TRY_COMPILE([#include <stdio.h>
+#include <sys/types.h>
 #include <sys/socket.h>],
                         [printf ("%d", SHUT_RD); return 0;],
                                         [shutrd=yes],
@@ -89,7 +90,8 @@ dnl ----------------------------------------------------------------------
 
 AC_CACHE_CHECK([for h_errno],
         herrno, [
-                AC_TRY_COMPILE([#include <netdb.h>],
+                AC_TRY_COMPILE([#include <stdio.h>
+#include <netdb.h>],
                         [printf ("%d", h_errno); return 0;],
                                         [herrno=yes],
                                         [herrno=no]),