Avoid implicit ints and implicit function declarations, to prevent
build failures with future compilers.

It's not clear if old compilers really use <stdio.h> for the exit
function.  Most of these checks would be better served by using
#error, so that they can use AC_TRY_LINK and be compatible with
cross-compilation.

Submitted upstream:

<https://lists.ibr.cs.tu-bs.de/hyperkitty/list/libsmi@ibr.cs.tu-bs.de/thread/MI6XPAR7JE2AY6UZIMXA3Q7JGD635JRR/>

diff --git a/configure.in b/configure.in
index 9b678e2d915a789d..c032538163bd05ab 100644
--- a/configure.in
+++ b/configure.in
@@ -42,8 +42,8 @@ AC_ARG_WITH(cc,
 CC="$withval")
 
 AC_MSG_CHECKING([for additional required compiler flags])
-AC_TRY_RUN([#include <stdio.h>
-  main() {
+AC_TRY_RUN([#include <stdlib.h>
+  int main(void) {
   #ifdef __SUNPRO_C
         exit(0);
   #else
@@ -100,8 +100,8 @@ dirseparator="$withval", dirseparator="")
 
 AC_MSG_CHECKING([for path separator character])
 if test "$pathseparator" = "" ; then
-  AC_TRY_RUN([#include <stdio.h>
-    main() {
+  AC_TRY_RUN([#include <stdlib.h>
+    int main(void) {
     #ifdef _WIN32
 	  exit(0);
     #else
@@ -114,8 +114,8 @@ echo $pathseparator
 
 AC_MSG_CHECKING([for dir separator character])
 if test "$dirseparator" = "" ; then
-  AC_TRY_RUN([#include <stdio.h>
-    main() {
+  AC_TRY_RUN([#include <stdlib.h>
+    int main(void) {
     #ifdef _WIN32
 	  exit(0);
     #else
@@ -140,8 +140,8 @@ AC_MSG_CHECKING([for 64 bit types])
 # "warning: decimal constant is so large that it is unsigned"
 # if used with the (correct) value -9223372036854775808LL.
 #
-AC_TRY_RUN([#include <stdio.h>
-      main() {
+AC_TRY_RUN([#include <stdlib.h>
+      int main(void) {
 	  long long ll;
 	  unsigned long long ull;
 	  exit(0);