Status of __STDC_IEC_559__ with modern C compilers

Question

I believe __STDC_IEC_559__ relies on some library features and can’t be defined solely by the compiler. See this post for some information. This is not uncommon for C — the compiler and the C library must sometimes cooperate in order to implement the entire standard.
What you’re asking depends on the compiler. I think you would have to have special knowledge of the compiler in order to decide this. In the specific case of GCC, it defines a macro to tell you. Search this node of the manual for __GCC_IEC_559.
Well… I don’t know the answer to this one :-). The original post seems to indicate that, yes, GCC might define __GCC_IEC_559 if it intends to implement IEEE 754, even if it does not actually do so.