autoconf: Fix C99 compatibility issue in the VA_COPY checks

The exit function is called without including <stdlib.h>, resulting
in an implicit function declarations.  Future compilers will not
support implicit function declarations by default.  This will lead
to build failures.

autoconf/configure.ac

@@ -400,6 +400,7 @@ AC_MSG_CHECKING(for an implementation of va_copy())
 AC_CACHE_VAL(slrn_cv_va_copy,[
 	AC_RUN_IFELSE([AC_LANG_SOURCE([[
 	#include <stdarg.h>
+	#include <stdlib.h>
 	void f (int i, ...) {
 	va_list args1, args2;
 	va_start (args1, i);

configure

@@ -8581,6 +8581,7 @@ else
 /* end confdefs.h.  */
 
 	#include <stdarg.h>
+	#include <stdlib.h>
 	void f (int i, ...) {
 	va_list args1, args2;
 	va_start (args1, i);
@@ -8626,6 +8627,7 @@ else
 /* end confdefs.h.  */
 
 	#include <stdarg.h>
+	#include <stdlib.h>
 	void f (int i, ...) {
 	va_list args1, args2;
 	va_start (args1, i);
@@ -8671,6 +8673,7 @@ else
 /* end confdefs.h.  */
 
 	#include <stdarg.h>
+	#include <stdlib.h>
 	void f (int i, ...) {
 	va_list args1, args2;
 	va_start (args1, i);