This local change was lost during the fixing of issue17192 to update
libffi to 3.0.13. (i'm not sure if it is needed anymore but see
issue 10309 for details which makes no mention of upstream)
diff --git a/Modules/_ctypes/libffi/src/dlmalloc.c b/Modules/_ctypes/libffi/src/dlmalloc.c
index 5c9f9c2..2773953 100644
--- a/Modules/_ctypes/libffi/src/dlmalloc.c
+++ b/Modules/_ctypes/libffi/src/dlmalloc.c
@@ -457,6 +457,11 @@
#define LACKS_ERRNO_H
#define MALLOC_FAILURE_ACTION
#define MMAP_CLEARS 0 /* WINCE and some others apparently don't clear */
+#elif !defined _GNU_SOURCE
+/* mremap() on Linux requires this via sys/mman.h
+ * See roundup issue 10309
+ */
+#define _GNU_SOURCE 1
#endif /* WIN32 */
#ifdef __OS2__