Trac #30594: ecl build with Xcode 12

The `configure` script for `ecl` fails with Xcode 12, with this kind of
error in `config.log`:
{{{
conftest.c:4:3: error: implicitly declaring library function 'exit' with
type 'void (int) __attribute__((noreturn))' [-Werror,-Wimplicit-
function-declaration]
  exit(0);
  ^
conftest.c:4:3: note: include the header <stdlib.h> or explicitly
provide a declaration for 'exit'
1 error generated.
}}}

upstream, in the develop branch, this is fixed in https://gitlab.com
/embeddable-common-lisp/ecl/-/merge_requests/231

URL: https://trac.sagemath.org/30594
Reported by: jhpalmieri
Ticket author(s): John Palmieri
Reviewer(s): Dima Pasechnik

build/pkgs/ecl/patches/ecl-configure-include-stdlib-h.patch

@@ -0,0 +1,42 @@
+--- a/src/configure	2020-04-24 03:54:52.000000000 -0700
++++ b/src/configure	2020-09-17 14:47:48.000000000 -0700
+@@ -2046,6 +2046,7 @@
+   cat confdefs.h - <<_ACEOF >conftest.$ac_ext
+ /* end confdefs.h.  */
+ $4
++#include <stdlib.h>
+ int
+ main ()
+ {
+@@ -2059,6 +2060,7 @@
+   cat confdefs.h - <<_ACEOF >conftest.$ac_ext
+ /* end confdefs.h.  */
+ $4
++#include <stdlib.h>
+ int
+ main ()
+ {
+@@ -7421,6 +7423,7 @@
+   cat confdefs.h - <<_ACEOF >conftest.$ac_ext
+ /* end confdefs.h.  */
+ #include <stdio.h>
++#include <stdlib.h>
+ int main() {
+   const char *int_type;
+   int bits;
+@@ -7709,6 +7712,7 @@
+   cat confdefs.h - <<_ACEOF >conftest.$ac_ext
+ /* end confdefs.h.  */
+ #include <stdio.h>
++#include <stdlib.h>
+ int main() {
+   const char *int_type;
+   int bits;
+@@ -8004,6 +8008,7 @@
+   cat confdefs.h - <<_ACEOF >conftest.$ac_ext
+ /* end confdefs.h.  */
+ #include <stdio.h>
++#include <stdlib.h>
+ int main() {
+   FILE *f = fopen("conftestval","w");
+   int c1, c2;