Unfortunately, it's not available in the bionic version of libc at all, which is what we use from the NDK. The alternative to fetching it dynamically would be to provide a default value on ifndef IOV_MAX, but I felt a dynamic fetch via sysconf was more flexible.

There's references to a default definition in some prebuilt components here, but this is as far as I can tell a fallback system for gcc internals:
http://androidxref.com/4.2.2_r1/search?q=IOV_MAX&defs=&refs=&path=&hist=&project=prebuilts

Other external packages that rely on the variable define it themselves:
http://androidxref.com/4.2.2_r1/search?q=IOV_MAX&defs=&refs=&path=&hist=&project=external