Fix symbol export with MSVC when features are disabled,
such as GLib. We need to generate the list of exported
symbols on the fly to make sure we only export symbols
that are actually available.
Needs some minor modifications to the gen-def.py script:
- accept header list also via command line args; we can't
pass things to a configure_file() command via the environment
in Meson.
- strip any leading 'src/' from library filename. This might
be there because in Meson the script might be called from
the top-level directory and not the current source directory.
Remove .def files again which had been checked in for earlier
versions of the Meson port.
Tim-Philipp Müller
Nirbheek Chauhan
Mathieu Duponchelle