Lib.rs

This crate is a hack so build.rs for cxx-qt-lib and cxx-qt-build both have access to cxx-qt-lib's C++ headers. This must be a separate crate from cxx-qt-lib because cxx-qt-lib cannot be a build dependency of cxx-qt-build. Otherwise Cargo links the executable compiled from a build.rs that uses cxx-qt-build to Qt, so running build.rs fails when Qt is linked dynamically if the Qt libraries are not in PATH (Windows)/LD_LIBRARY_PATH (Unix).