Avoid implicit function declarations and an implicit int type for C99
compatibility.

Submitted upstream: <https://github.com/NagiosEnterprises/nrpe/pull/273>

diff --git a/configure b/configure
index 215fec35be70c102..8c6c1e6dbca8abad 100755
--- a/configure
+++ b/configure
@@ -6934,6 +6934,9 @@ else
 
 #include <sys/types.h>
 #include <stdarg.h>
+#include <stdio.h>
+#include <stdlib.h>
+#include <string.h>
 void foo(const char *format, ...) {
 	va_list ap;
 	int len;
@@ -6953,7 +6956,7 @@ void foo(const char *format, ...) {
 
 	exit(0);
 }
-main() { foo("hello"); }
+int main() { foo("hello"); }
 
 _ACEOF
 if ac_fn_c_try_run "$LINENO"; then :
diff --git a/configure.ac b/configure.ac
index 1ef4b5af5a55d7fe..6166c7423c97db81 100644
--- a/configure.ac
+++ b/configure.ac
@@ -203,6 +203,9 @@ AC_CACHE_CHECK([for C99 vsnprintf],ac_cv_HAVE_C99_VSNPRINTF,[
 AC_TRY_RUN([
 #include <sys/types.h>
 #include <stdarg.h>
+#include <stdio.h>
+#include <stdlib.h>
+#include <string.h>
 void foo(const char *format, ...) {
 	va_list ap;
 	int len;
@@ -222,7 +225,7 @@ void foo(const char *format, ...) {
 
 	exit(0);
 }
-main() { foo("hello"); }
+int main() { foo("hello"); }
 ],
 ac_cv_HAVE_C99_VSNPRINTF=yes,ac_cv_HAVE_C99_VSNPRINTF=no,ac_cv_HAVE_C99_VSNPRINTF=cross)])
 if test x"$ac_cv_HAVE_C99_VSNPRINTF" = x"yes"; then