Avoid implicit function declarations, a C feature removed in 1999.

diff --git a/aclocal.m4 b/aclocal.m4
index a357ef74b30a09a6..2d2af334d5369e8e 100644
--- a/aclocal.m4
+++ b/aclocal.m4
@@ -6727,6 +6727,7 @@ AC_DEFUN(AC_LIBELF_SXWORD,
 #include <stdlib.h>
 #include <stdio.h>
 #include <string.h>
+#include <unistd.h>
 
 static unsigned char sparc64_elf[] = {
 0x7f,0x45,0x4c,0x46,0x02,0x02,0x01,0x00,0x00,0x00,0x00,0x00,0x00,0x00,0x00,0x00,
diff --git a/configure b/configure
index 9f2a746fbf3f7471..960e140559761417 100755
--- a/configure
+++ b/configure
@@ -4438,8 +4438,8 @@ main ()
   for (i = 0; i < 256; i++)
     if (XOR (islower (i), ISLOWER (i))
         || toupper (i) != TOUPPER (i))
-      exit(2);
-  exit (0);
+      return 2;
+  return 0;
 }
 _ACEOF
 rm -f conftest$ac_exeext
@@ -19411,6 +19411,7 @@ cat >>conftest.$ac_ext <<_ACEOF
 #include <stdlib.h>
 #include <stdio.h>
 #include <string.h>
+#include <unistd.h>
 
 static unsigned char sparc64_elf[] = {
 0x7f,0x45,0x4c,0x46,0x02,0x02,0x01,0x00,0x00,0x00,0x00,0x00,0x00,0x00,0x00,0x00,