Only try to use F_SETPIPE_SZ if it's available
Only try to use F_SETPIPE_SZ if it's available

Summary:
As the comment says, we can ignore errors in setting the size.
So it stands to reason we can ignore setting the size as well.

Test Plan: HHVM's `cmake build verification`

Reviewed By: yfeldblum@fb.com

Subscribers: fugalh, folly-diffs@, jsedgwick, yfeldblum, chalfant

FB internal diff: D2242882

Signature: t1:2242882:1436923555:0aa428fe218f1d16dc63b720593a82746c8902f3

facebook/wangle,facebook/wangle,mzhaom/wangle,Dalzhim/wangle,jamperry/wangle,Orvid/wangle,AresTao/wangle,facebook/wangle,AresTao/wangle,raphaelamorim/wangle,Orvid/wangle,mzhaom/wangle,raphaelamorim/wangle,Dalzhim/wangle,jamperry/wangle