Fix C99 compatibility issues (implicit int, implicit function
declarations) in the configure script.  Upstream has removed
autotools altogether, addressing this issue:

commit 5ea5d5ec7c7f4e628db5f0b4c446dc510ff2777a
Author: Brian Stafford <contact@brianstafford.info>
Date:   Tue Mar 3 18:40:30 2020 +0000

    Remove autotools
    
    * it has only been a 20 year wait for this :-)

diff --git a/acinclude.m4 b/acinclude.m4
index 912149736f74a70b..3632220c1a107dd1 100644
--- a/acinclude.m4
+++ b/acinclude.m4
@@ -355,12 +355,12 @@ AC_DEFUN([ACX_SNPRINTF], [
 	AC_TRY_RUN([
 #include <stdio.h> 
 
-main ()
+int main (void)
 {
   char buf[16];
 
   snprintf (buf, 4, "abcd");
-  exit ((buf[3] == '\0') ? 0 : 1);
+  return (buf[3] == '\0') ? 0 : 1;
 }
 
 ], acx_cv_working_snprintf=yes, acx_cv_working_snprintf=no, acx_cv_working_snprintf=yes)
diff --git a/configure b/configure
index d586ca2df4fc0435..f5e13a62ef13a55d 100755
--- a/configure
+++ b/configure
@@ -13211,13 +13211,14 @@ else
 /* end confdefs.h.  */
 
 #include <stdio.h>
+#include <string.h>
 
 main ()
 {
   char buf[16];
 
   snprintf (buf, 4, "abcd");
-  exit ((buf[3] == '\0') ? 0 : 1);
+  return (buf[3] == '\0') ? 0 : 1;
 }