Without having reviewed the proposed change in detail, a couple of comments.  On current OS X systems and others, the compiler could be clang which perhaps should be treated as gcc for most autoconf purposes.  Also, why are we putting any effort into supporting IRIX anymore?  Both IRIX and the MIPS processor lines it runs on was retired by SGI in 2007.  That's why support for IRIX was pulled from Python 3.  It's a dead end.