conditional compilation for __attribute__((nonstring))

clang-20 complains by default about unrecognized attributes, so only
include nonstring for >= 21

Do the same for gcc <10, since we don't do any testing on those older
versions.

Author: mutability

ais_charset.c

@@ -1,3 +1,4 @@
 #include "ais_charset.h"
+#include "compat/compat.h"
 
-char ais_charset[64] __attribute__ ((nonstring)) = "@ABCDEFGHIJKLMNOPQRSTUVWXYZ[\\]^_ !\"#$%&'()*+,-./0123456789:;<=>?";
+char ais_charset[64] NONSTRING = "@ABCDEFGHIJKLMNOPQRSTUVWXYZ[\\]^_ !\"#$%&'()*+,-./0123456789:;<=>?";

compat/compat.h

@@ -5,6 +5,14 @@
  * Platform-specific bits
  */
 
+#if defined(__clang__) && __clang_major__ >= 21
+#  define NONSTRING __attribute__((nonstring))
+#elif defined(__GNUC__) && __GNUC__ >= 10
+#  define NONSTRING __attribute__((nonstring))
+#else
+#  define NONSTRING /* nothing */
+#endif
+
 #if defined(__APPLE__)
 
 /*

interactive.c

@@ -140,7 +140,7 @@ void interactiveShowData(void) {
     static bool need_clear = true;
     uint64_t now = mstime();
     char progress;
-    char spinner[4] __attribute__ ((nonstring)) = "|/-\\";
+    char spinner[4] NONSTRING = "|/-\\";
     int valid = 0;
     double signalMax = -100.0;
     double signalMin = +100.0;