IAR comiler for ARM errornously reports __has_attribute(nonnull) true#46

The IAR compiler appears to by lying about attribute support: ``__has_attribute(nonnull)``, ``__has_attribute(const)`` and ``__has_attribute(pure)`` are falsely reporting existing compiler support for these attributes (which leads to warnings when the resulting code is compiled). ``Warning[Pa167]: the "__nonnull__" attribute is not supported`` I'd suggest to suppress the attribute usage when using the IAR compiler for these attributes like so: ```C++ #if defined(JSON_HEDLEY_NON_NULL) #undef JSON_HEDLEY_NON_NULL #endif #if \ !defined(JSON_HEDLEY_IAR_VERSION) && \ JSON_HEDLEY_HAS_ATTRIBUTE(nonnull) || \ JSON_HEDLEY_GCC_VERSION_CHECK(...) || \ ... #define JSON_HEDLEY_NON_NULL(...) __attribute__((__nonnull__(__VA_ARGS__))) #else #define JSON_HEDLEY_NON_NULL(...) #endif ```