Added a libprocess environment variable for
enabling the cpu profiler.

The perftools profiler causes crashes on 64 bit systems with
libunwind or gcc < 2.12. This is to remove access to the profiler,
unless explicitly overridden in the environment.

From: Ben Mahler <benjamin.mahler@gmail.com>
Review: https://reviews.apache.org/r/9583

git-svn-id: https://svn.apache.org/repos/asf/incubator/mesos/trunk@1449875 13f79535-47bb-0310-9956-ffa450edef68