Track C++ stdlib in use

Right now the C++ stdlib tracking assumes that every package uses C++, which results in some false positives when trying to link clang-compiled code with GCC-compiled code. This adds a new check in the postinstall step which checks to see whether the software actually linked against a C++ stdlib, so that homebrew will only complain on install if a dependency *actually* has an incompatible C++ stdlib.

Ping @manphiz