Work around pipe2 on non-linux
Work around pipe2 on non-linux

Use pipe instead of pipe2 on non-linux.
Allows problemtools to compile on OS X.

jsannemo/problemtools,Kattis/problemtools,jsannemo/problemtools,Kattis/problemtools,ghamerly/problemtools,ghamerly/problemtools,Kattis/problemtools,Kattis/problemtools,jsannemo/problemtools,jsannemo/problemtools,ghamerly/problemtools,ghamerly/problemtools