getaddrinfo() AI_IDN feature test

[Martin Lambers]

Dear glibc folks,

I recently changed two programs to use the AI_IDN flag for
getaddrinfo() instead of using libidn.

Users reported runtime failure (EAI_BADFLAGS) on systems where glibc
was build without libidn support.

On these systems, AI_IDN was defined nevertheless -- is this intended
behaviour? If so, then #ifdef AI_IDN is not enough; what is the
recommended way to check if AI_IDN actually works?

Best regards,
Martin