sscanf needs <stdio.h> for the prototype.  Submitted upstream here:

  <https://support.zabbix.com/browse/ZBX-21946>

diff --git a/configure.ac b/configure.ac
index 0588004f9f89cdd5..bbc60e3a28369f9f 100644
--- a/configure.ac
+++ b/configure.ac
@@ -952,6 +952,7 @@ dnl FreeBSD 4.x does not support %llu
 AC_MSG_CHECKING(for long long format)
 AC_RUN_IFELSE([AC_LANG_SOURCE([[
 #include <sys/types.h>
+#include <stdio.h>
 int main()
 {
         uint64_t i;