Fix phpGH-20492: mbstring compile warning due to non-strings

This is a partial backport of ea69276, but without changing public
headers as that's not allowed at this point.

Closes phpGH-20494.

Author: ndossche

NEWS

@@ -37,6 +37,8 @@ PHP                                                                        NEWS
 - MbString:
   . Fixed bug GH-20491 (SLES15 compile error with mbstring oniguruma).
     (ndossche)
+  . Fixed bug GH-20492 (mbstring compile warning due to non-strings).
+    (ndossche)
 
 - Opcache:
   . Fixed bug GH-20329 (opcache.file_cache broken with full interned string

ext/mbstring/libmbfl/filters/mbfilter_cjk.c

@@ -21,6 +21,13 @@
  * This macro converts uppercase ASCII values to Regional Indicator codepoints */
 #define NFLAGS(c) (0x1F1A5+((unsigned int)(c)))
 
+#if __has_attribute(nonstring) && defined(__GNUC__) && ((!defined(__clang__) && __GNUC__ >= 15) || (defined(__clang_major__) && __clang_major__ >= 20))
+# define ZEND_NONSTRING __attribute__((nonstring))
+#else
+# define ZEND_NONSTRING
+#endif
+
+
 static const char nflags_s[10][2] ZEND_NONSTRING = {"CN", "DE", "ES", "FR", "GB", "IT", "JP", "KR", "RU", "US"};
 static const int nflags_code_kddi[10] = { 0x2549, 0x2546, 0x24C0, 0x2545, 0x2548, 0x2547, 0x2750, 0x254A, 0x24C1, 0x27F7 };
 static const int nflags_code_sb[10] = { 0x2B0A, 0x2B05, 0x2B08, 0x2B04, 0x2B07, 0x2B06, 0x2B02, 0x2B0B, 0x2B09, 0x2B03 };