relicense everything under CC0-1.0 or Unlicense, add SPDX ID

c-programming/math/be_to_le_ptr.h

@@ -4,7 +4,8 @@
  * Useful for showing big-endian hexadecimal dumps on little-endian machines
  *
  * Author: Intel A80486DX2-66
- * License: Unlicense
+ * License: Creative Commons Zero 1.0 Universal or Unlicense
+ * SPDX-License-Identifier: CC0-1.0 OR Unlicense
  */
 
 #define BE_TO_LE_PTR(base, size, i)                                           \

c-programming/math/opt_int_div.h

@@ -6,7 +6,8 @@
  * NOTE: This code will work only on a computer that uses bits.
  *
  * Author: Intel A80486DX2-66
- * License: Unlicense
+ * License: Creative Commons Zero 1.0 Universal or Unlicense
+ * SPDX-License-Identifier: CC0-1.0 OR Unlicense
  */
 
 #ifndef _OPT_INT_DIV_H

c-programming/math/opt_int_div.test.c

@@ -4,7 +4,8 @@
  * "Optimized integer division"
  *
  * Author: Intel A80486DX2-66
- * License: Unlicense
+ * License: Creative Commons Zero 1.0 Universal or Unlicense
+ * SPDX-License-Identifier: CC0-1.0 OR Unlicense
  */
 
 #include "opt_int_div.h"