AmpGen: Small fixes for clang and libc++

These are a few fixes needed for libc++ to work again. In the future, seeking on std::cin should be removed, because it is undefined (and doesn't work on libc++, just on libstdc++ for now).

This also allows AMPGEN_CXX to be set at build time; needed for macOS since the CMake compiler path is the wrong one (it requires the system library path to be manually set, while the one at which c++ does not).

I've also activated clang-tidy to clean up the code a bit, doing things like using nullptr instead of 0.

@tevans, the next PR I'm planning will apply the official LHCb clang-format from Gaudi to clean up formatting issues that (mostly I) have introduced.