Fixed configure.ac to require libc++ only on Mac OS X.

This patch fixes configure failure on Linux clang builds.

Review: https://reviews.apache.org/r/16924
Fixed configure.ac to require libc++ only on Mac OS X.

This patch fixes configure failure on Linux clang builds.

Review: https://reviews.apache.org/r/16909
Fixed a bug that causes incorrect log output when there is no master
detected.

Review: https://reviews.apache.org/r/17338
Added a function that captures the stack trace when a pure virtual
method is called.

Review: https://reviews.apache.org/r/17221
Fixed flaky test GroupTest.GroupWatchWithSessionExpiration.

This test became flaky due to a recent change in MESOS-935 that made
Group update watchers in expired(), this introduced a race condition
on the 'membership.get().cancelled()' Future (Group thread sets it
and the test thread reads it).

Review: https://reviews.apache.org/r/17714
Group now aborts in the face of a ZNOAUTH error when creating the
parent path for the group.

See: https://issues.apache.org/jira/browse/MESOS-958

Review: https://reviews.apache.org/r/17577