add MISSING_HEADER hook #579

Adding a hook which provides data when MISSING_HEADER is encountered would allow Cppcheck a way to inject the macros the header defines without providing the whole code - see https://trac.cppcheck.net/ticket/13341. It would also allow for on-demand automatic library loading.

To work correctly this would also need to provide some context since the header evaluation is based on previously defined macros.