Install g++-4.8 even when compiling with Clang.
Install g++-4.8 even when compiling with Clang.

This is so that Clang can have a newer libstdc++; without it, Simbody's tests will fail.
opensim-org/opensim-core,opensim-org/opensim-core,opensim-org/opensim-core,opensim-org/opensim-core,opensim-org/opensim-core,opensim-org/opensim-core,opensim-org/opensim-core