scanner: Add a way to specify extra libraries to link against

Currently the only way to get a library to link against the dumper
program is through '--link' which implies that library will be defined
in the 'libraries' field of the .gir. When using libtool, we
link against dependencies of that library as the .la defines that, but
when using --no-libtool that won't happen and the user needs to be
able to define to what other libraries the program needs to be link
against, and this is what the new --extra-library argument is about.

https://bugzilla.gnome.org/show_bug.cgi?id=774625