Change remaining hash_set -> unordered_set (#208)

Change remaining hash_set -> unordered_set (#208)

The nightly competition build has been failing due to a remaining use of
hash_set in approx_simplex.cpp. This commit changes the remaining uses
of hash_set to unordered_set.
The remaining uses of hash_set were in LFSC. Switching to C++11 for LFSC
required changing the configure.ac for LFSC to require C++11 support to
make sure that it can be compiled independently from the rest of CVC4 (some
of our Travis tests do that as well). To have the macros for these additional
checks available, the commit adds a symlink to the files in config that contain
the macros). I did not find a way to add macros from a parent's folder that
did not break `make distcheck