diff '--exclude=include' -ruN mit-scheme-12.1/src/blowfish/blowfish.c mit-scheme-12.1-mod/src/blowfish/blowfish.c
--- mit-scheme-12.1/src/blowfish/blowfish.c	2020-07-04 08:46:25.000000000 +0300
+++ mit-scheme-12.1-mod/src/blowfish/blowfish.c	2026-10-24 18:31:16.826010462 +0300
@@ -24,7 +24,9 @@
  * SUCH DAMAGE.
  */
 
+#ifndef _POSIX_C_SOURCE
 #define	_POSIX_C_SOURCE	200809L
+#endif
 
 #include <assert.h>
 #include <stdint.h>
diff '--exclude=include' -ruN mit-scheme-12.1/src/microcode/chacha.i mit-scheme-12.1-mod/src/microcode/chacha.i
--- mit-scheme-12.1/src/microcode/chacha.i	2021-05-07 19:02:08.000000000 +0300
+++ mit-scheme-12.1-mod/src/microcode/chacha.i	2026-10-24 18:34:08.914012448 +0300
@@ -29,7 +29,9 @@
  * POSSIBILITY OF SUCH DAMAGE.
  */
 
+#ifndef _POSIX_C_SOURCE
 #define	_POSIX_C_SOURCE	200809L
+#endif
 
 #include "chacha.h"
 
diff '--exclude=include' -ruN mit-scheme-12.1/src/microcode/keccak.c mit-scheme-12.1-mod/src/microcode/keccak.c
--- mit-scheme-12.1/src/microcode/keccak.c	2017-09-12 06:34:42.000000000 +0300
+++ mit-scheme-12.1-mod/src/microcode/keccak.c	2026-10-24 18:33:03.363011691 +0300
@@ -24,7 +24,9 @@
  * SUCH DAMAGE.
  */
 
+#ifndef _POSIX_C_SOURCE
 #define	_POSIX_C_SOURCE	200809L
+#endif
 
 #include <stdint.h>
 
diff '--exclude=include' -ruN mit-scheme-12.1/src/microcode/md5.c mit-scheme-12.1-mod/src/microcode/md5.c
--- mit-scheme-12.1/src/microcode/md5.c	2019-01-11 12:36:56.000000000 +0300
+++ mit-scheme-12.1-mod/src/microcode/md5.c	2026-10-24 18:32:18.785011177 +0300
@@ -24,7 +24,9 @@
  * SUCH DAMAGE.
  */
 
+#ifndef _POSIX_C_SOURCE
 #define	_POSIX_C_SOURCE	200809L
+#endif
 
 #include <assert.h>
 #include <stdint.h>