Auto merge of #1319 - jasonbking:readdir, r=gnzlbg

This fixes issue #1318 - on Solarish systems __posix_readdir_r is only

defined for 32-bit Solarish targets.  Since rust doesn't currently support 32-bit Solarish targets, the line is both unnecessary and wrong.

As found with the latest fd binary, removing the line fixed the fd build (and fd then works as expected).