support GCC15

GCC 15 starts warning about non NUL-terminated string literals:

    chacha.c:44:31: error: initializer-string for array of ‘char’ truncates NUL terminator but destination lacks ‘nonstring’ attribute (17 chars into 16 available) [-Werror=unterminated-string-initialization]
       44 | static const char sigma[16] = "expand 32-byte k";
          |                               ^~~~~~~~~~~~~~~~~~

Author: cgzones

chacha.c

@@ -41,7 +41,7 @@ static const unsigned rounds = 8;
     a = PLUS(a, b); d = ROTATE(XOR(d, a), 8); \
     c = PLUS(c, d); b = ROTATE(XOR(b, c), 7);
 
-static const char sigma[16] = "expand 32-byte k";
+static const char sigma[16] NONSTRING = "expand 32-byte k";
 
 void chacha_keysetup(chacha_ctx *x, const u8 *k) {
     x->input[0] = U8TO32_LITTLE(sigma + 0);

util.h

@@ -32,6 +32,17 @@
 #define STRINGIFY(s) #s
 #define ALIAS(f) __attribute__((alias(STRINGIFY(f))))
 
+// supported since GCC 15
+#if defined __has_attribute
+#  if __has_attribute (nonstring)
+#    define NONSTRING __attribute__ ((nonstring))
+#  else
+#    define NONSTRING
+#  endif
+#else
+#  define NONSTRING
+#endif
+
 typedef uint8_t u8;
 typedef uint16_t u16;
 typedef uint32_t u32;