Gentoo issue: https://bugs.gentoo.org/899796

diff -Naur a/configure.ac b/configure.ac
--- a/configure.ac
+++ b/configure.ac
@@ -1291,13 +1291,14 @@
 	#include <sys/types.h>
 	#include <sys/ipc.h>
 	#include <sys/shm.h>
-	int main()
+	#include <stdlib.h>
+	int main(void)
 	{
 	  int id;
 	  char *shmaddr;
-	id = shmget (IPC_PRIVATE, 4, IPC_CREAT | 0600);
-	if (id == -1)
-	  exit (2);
+	  id = shmget (IPC_PRIVATE, 4, IPC_CREAT | 0600);
+	  if (id == -1)
+	    exit (2);
 	  shmaddr = shmat (id, 0, 0);
 	  shmctl (id, IPC_RMID, 0);
 	  if ((char*) shmat (id, 0, 0) == (char*) -1)