[cmake] Add a big warning about a libstdc++ issue

Summary:
Recent increase in the usage of std::weak_ptr has caused us to rediscover an issue in libstdc++
versions prior to 4.9 <https://gcc.gnu.org/bugzilla/show_bug.cgi?id=59656>, which make this class
unusable without exceptions in the presence of multiple threads. It's virtualy impossible to work
around this issue without implementing our own shared_ptr/weak_ptr substitutes, which does not
seem like a good idea.

Therefore, I am adding a big CMake warning which warns you about this issue if you're attempting
a to do a build which is suceptible to this problem and suggests possible alternatives. Right
now, nothing spectacular will happen if you ignore this warning (all the crashes I have seen
occur during process shutdown), but there's no guarantee the situation will not change in the
future.

Reviewers: tberghammer, tfiala, nitesh.jain, omjavaid, emaste, krytarowski

Subscribers: lldb-commits

Differential Revision: http://reviews.llvm.org/D20671

llvm-svn: 270854