builds now require gcc 4.8+ or clang 3.5+

[Benjamin Hindman]

I'm happy to announce that we've committed the updates to configure.ac that
require gcc 4.8+ and clang 3.5+! See this thread
<https://mail-archives.apache.org/mod_mbox/mesos-dev/201504.mbox/%3CCAPkqSi9aJLi2VeLUTEBrdh4wzvHdpn0TvmffjOQGYnnB8DEDSA%40mail.gmail.com%3E>
for
the discussion. Stay tuned for reviews updating the style guide with our
plans for using C++11 features. It's been a long time coming!