src: wrap MIN definition in infdef

Some platforms already define this; avoid redefining if that's
the case. Found on OpenBSD 5.6.

PR-URL: https://github.com/iojs/io.js/pull/1322


Reviewed-By: Brian White <mscdex@mscdex.net>
Reviewed-By: Ben Noordhuis <info@bnoordhuis.nl>