Most of the issues are historic problems in autoconf 2.13, but the
memrchr change is package-specific and needs to be upstreamed.

Furthermore, upstream still seems to be using autoconf 2.13 for
new releases, so until they switch, we'll have to keep patching this
downstream.

diff --git a/configure b/configure
index 477d84bfd3092e41..3562cc6a3d9a09b0 100755
--- a/configure
+++ b/configure
@@ -995,7 +995,7 @@ cat > conftest.$ac_ext << EOF
 #line 998 "configure"
 #include "confdefs.h"
 
-main(){return(0);}
+int main(){return(0);}
 EOF
 if { (eval echo configure:1003: \"$ac_link\") 1>&5; (eval $ac_link) 2>&5; } && test -s conftest${ac_exeext}; then
   ac_cv_prog_cc_works=yes
@@ -1258,8 +1258,8 @@ else
 #define TOUPPER(c) (ISLOWER(c) ? 'A' + ((c) - 'a') : (c))
 #define XOR(e, f) (((e) && !(f)) || (!(e) && (f)))
 int main () { int i; for (i = 0; i < 256; i++)
-if (XOR (islower (i), ISLOWER (i)) || toupper (i) != TOUPPER (i)) exit(2);
-exit (0); }
+if (XOR (islower (i), ISLOWER (i)) || toupper (i) != TOUPPER (i)) return 2;
+return 0; }
 
 EOF
 if { (eval echo configure:1268: \"$ac_link\") 1>&5; (eval $ac_link) 2>&5; } && test -s conftest${ac_exeext} && (./conftest; exit) 2>/dev/null
@@ -2044,7 +2044,7 @@ for ac_kw in inline __inline__ __inline; do
 #include "confdefs.h"
 
 int main() {
-} $ac_kw foo() {
+} $ac_kw int foo() {
 ; return 0; }
 EOF
 if { (eval echo configure:2053: \"$ac_compile\") 1>&5; (eval $ac_compile) 2>&5; }; then
@@ -2311,12 +2311,12 @@ else
 #include "confdefs.h"
 #include <stdio.h>
 #include <sys/types.h>
-main()
+int main(void)
 {
   FILE *f=fopen("conftestval", "w");
-  if (!f) exit(1);
+  if (!f) return 1;
   fprintf(f, "%d\n", sizeof(unsigned short));
-  exit(0);
+  return 0;
 }
 EOF
 if { (eval echo configure:2325: \"$ac_link\") 1>&5; (eval $ac_link) 2>&5; } && test -s conftest${ac_exeext} && (./conftest; exit) 2>/dev/null
@@ -2351,12 +2351,12 @@ else
 #include "confdefs.h"
 #include <stdio.h>
 #include <sys/types.h>
-main()
+int main(void)
 {
   FILE *f=fopen("conftestval", "w");
-  if (!f) exit(1);
+  if (!f) return 1;
   fprintf(f, "%d\n", sizeof(unsigned));
-  exit(0);
+  return 0;
 }
 EOF
 if { (eval echo configure:2365: \"$ac_link\") 1>&5; (eval $ac_link) 2>&5; } && test -s conftest${ac_exeext} && (./conftest; exit) 2>/dev/null
@@ -2391,12 +2391,12 @@ else
 #include "confdefs.h"
 #include <stdio.h>
 #include <sys/types.h>
-main()
+int main(void)
 {
   FILE *f=fopen("conftestval", "w");
-  if (!f) exit(1);
+  if (!f) return 1;
   fprintf(f, "%d\n", sizeof(unsigned long));
-  exit(0);
+  return 0;
 }
 EOF
 if { (eval echo configure:2405: \"$ac_link\") 1>&5; (eval $ac_link) 2>&5; } && test -s conftest${ac_exeext} && (./conftest; exit) 2>/dev/null
@@ -2432,12 +2432,12 @@ else
 #include "confdefs.h"
 #include <stdio.h>
 #include <sys/types.h>
-main()
+int main(void)
 {
   FILE *f=fopen("conftestval", "w");
-  if (!f) exit(1);
+  if (!f) return 1;
   fprintf(f, "%d\n", sizeof(unsigned long long));
-  exit(0);
+  return 0;
 }
 EOF
 if { (eval echo configure:2446: \"$ac_link\") 1>&5; (eval $ac_link) 2>&5; } && test -s conftest${ac_exeext} && (./conftest; exit) 2>/dev/null
@@ -4033,6 +4033,7 @@ else
   cat > conftest.$ac_ext <<EOF
 #line 4037 "configure"
 #include "confdefs.h"
+#define _GNU_SOURCE
 #include <string.h>
 int main() {
 char * volatile str = ""; volatile int l = 1; return !!memrchr(str, 0, l);
@@ -8933,6 +8934,8 @@ cat > conftest.$ac_ext <<EOF
 #line 8932 "configure"
 #include "confdefs.h"
 
+char ${x_direct_test_function}(void);
+
 int main() {
 ${x_direct_test_function}()
 ; return 0; }