core/compiler_hints: Add NONSTRING attribute

This adds the `NONSTRING` attribute that is defined as either
`__attribute__((nonstring))` or as empty, depending on whether the
toolchain understands the `nonstring` attribute.

This allows declaring char arrays as not being a zero-terminated
c-string without cluttering the code with preprocessor conditinational
to ensure backward compatibility with compilers that do not support
`-Wunterminated-string-initialization` yet.

Author: maribu

core/lib/include/compiler_hints.h

@@ -40,6 +40,20 @@ extern "C" {
 #  endif
 #endif
 
+/**
+ * @def NONSTRING
+ * @brief The `NONSTRING` keyword tells the compiler to assume that a char array
+ *        is not used as c string. (Specifically: It does not need a terminating
+ *        zero byte.)
+ */
+#ifndef NONSTRING
+#  if (__GNUC__ >= 15)
+#    define NONSTRING __attribute__((nonstring))
+#  else
+#    define NONSTRING
+#  endif
+#endif
+
 /**
  * @def PURE
  * @brief The function has no effects except the return value and its return