Problem: priority_queue-t fails to build with error:
warning: assuming signed overflow does not occur when assuming that
(X - c) > X is always false [-Wstrict-overflow]

This *only* happens when building with -DCMAKE_CXX_FLAGS=-O2
(not with -O1 or -O3), so it looks like a compiler optmization bug
(gcc version 5.1.1)

Fix: remove the constant, use random number instead.