During the exp-run in bug 208158, it was found that dns/powerdns gives
errors with libc++ 3.8.0:

dnspacket.cc:645:6: error: call to 'abs' is ambiguous

if(abs(trc->d_time - now) > trc->d_fudge) {
   ^~~

This is because abs() is being called with unsigned arguments. Import
upstream commit f2d05dd to fix it.

Approved by: tremere@cainites.net
PR: 208725
MFH: 2016Q2