[build] do not output errors when pid_max does not exist
[build] do not output errors when pid_max does not exist

Summary: This was spamming the build output on osx.

Reviewed By: ezgicicek

Differential Revision: D23190091

fbshipit-source-id: 9ad33655f

facebook/infer,jvillard/infer,jvillard/infer,jvillard/infer,facebook/infer,jvillard/infer,facebook/infer,facebook/infer,facebook/infer,facebook/infer,jvillard/infer,jvillard/infer,jvillard/infer,facebook/infer